drm/amd/display: Check if Mode is Supported Before Returning Result

drm/amd/display: Check if Mode is Supported Before Returning Result

[Why]
Even if the mode is not supported dml2_check_mode_supported() would still return true.
This causes an unsupported mode to be programmed.

[How]
Check if the mode is supported or not and return the proper result.

Reviewed-by: Chaitanya Dhere <chaitanya.dhere@amd.com>
Signed-off-by: Austin Zheng <austin.zheng@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>