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

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.

Cc: stable@vger.kernel.org
Fixes: dd86dc2f9ae1 ("drm/sti: implement atomic_check for the planes")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Acked-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240909063359.1197065-1-make24@iscas.ac.cn
Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>