drm/sti: avoid potential dereference of error pointers in sti_gdp_atomic_check

In the Linux kernel, the following vulnerability has been resolved:

drm/sti: avoid potential dereference of error pointers in sti_gdp_atomic_check

The return value of drm_atomic_get_crtc_state() needs to be

checked. To avoid use of error pointer 'crtc_state' in case

of the failure.