drm/amd/display: Fix NULL pointer dereference in dmub_tracebuffer_show

drm/amd/display: Fix NULL pointer dereference in dmub_tracebuffer_show

It corrects the issue by checking if 'adev->dm.dmub_srv' is NULL before
accessing its 'meta_info' member. This ensures that we do not
dereference a NULL pointer.

Fixes the below:
	drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_debugfs.c:917 dmub_tracebuffer_show()
	warn: address of 'adev->dm.dmub_srv->meta_info' is non-NULL

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_debugfs.c
    901 static int dmub_tracebuffer_show(struct seq_file *m, void *data)
    902 {
    903         struct amdgpu_device *adev = m->private;
    904         struct dmub_srv_fb_info *fb_info = adev->dm.dmub_fb_info;
    905         struct dmub_fw_meta_info *fw_meta_info = &adev->dm.dmub_srv->meta_info;
                                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Even if adev->dm.dmub_srv is NULL, the address of ->meta_info can't be NULL

    906         struct dmub_debugfs_trace_entry *entries;
    907         uint8_t *tbuf_base;
    908         uint32_t tbuf_size, max_entries, num_entries, first_entry, i;
    909
    910         if (!fb_info)
    911                 return 0;
    912
    913         tbuf_base = (uint8_t *)fb_info->fb[DMUB_WINDOW_5_TRACEBUFF].cpu_addr;
    914         if (!tbuf_base)
    915                 return 0;
    916
--> 917         tbuf_size = fw_meta_info ? fw_meta_info->trace_buffer_size :
                            ^^^^^^^^^^^^ Always non-NULL

    918                                    DMUB_TRACE_BUFFER_SIZE;
    919         max_entries = (tbuf_size - sizeof(struct dmub_debugfs_trace_header)) /
    920                       sizeof(struct dmub_debugfs_trace_entry);
    921
    922         num_entries =

v2: Initialize struct dmub_fw_meta_info *fw_meta_info to NULL (Dan Carpenter)

Fixes: 5a498172c8d0 ("drm/amd/display: Make DMCUB tracebuffer debugfs chronological")
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>