drm/amd/display: Add NULL pointer checks in dm_force_atomic_commit()

drm/amd/display: Add NULL pointer checks in dm_force_atomic_commit()

This commit updates the dm_force_atomic_commit function to replace the
usage of PTR_ERR_OR_ZERO with IS_ERR for checking error states after
retrieving the Connector (drm_atomic_get_connector_state), CRTC
(drm_atomic_get_crtc_state), and Plane (drm_atomic_get_plane_state)
states.

The function utilized PTR_ERR_OR_ZERO for error checking. However, this
approach is inappropriate in this context because the respective
functions do not return NULL; they return pointers that encode errors.

This change ensures that error pointers are properly checked using
IS_ERR before attempting to dereference.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>