drm/vc4: kms: Fix IS_ERR() vs NULL check for vc4_kms

drm/vc4: kms: Fix IS_ERR() vs NULL check for vc4_kms

The drm_atomic_get_new_private_obj_state() function returns NULL
on error path, drm_atomic_get_old_private_obj_state() function
returns NULL on error path, too, they does not return error pointers.

By the way, vc4_hvs_get_new/old_global_state() should return
ERR_PTR(-EINVAL), otherwise there will be null-ptr-defer issue,
such as follows:

In function vc4_atomic_commit_tail():
  |-- old_hvs_state = vc4_hvs_get_old_global_state(state); <-- return NULL
  |-- if (WARN_ON(IS_ERR(old_hvs_state))) <-- no return
  |-- unsigned long state_rate = max(old_hvs_state->core_clock_rate,
	new_hvs_state->core_clock_rate); <-- null-ptr-defer

Fixes: 9ec03d7f1ed3 ("drm/vc4: kms: Wait on previous FIFO users before a commit")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20221110094445.2930509-6-cuigaosheng1@huawei.com