Fix the warning
"warn: variable dereferenced before check 'crtc' (see line 390)"
by removing unnecessary checks as ltdc_crtc_update_clut() is
only called from ltdc_crtc_atomic_flush() where crtc and
crtc->state are not NULL.
Many thanks to Dan Carpenter for the bug report
https://lists.freedesktop.org/archives/dri-devel/2018-February/166918.html