drm/stm: ltdc: fix warning in ltdc_crtc_update_clut()

drm/stm: ltdc: fix warning in ltdc_crtc_update_clut()

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

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: yannick fertre <yannick.fertre@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180410135312.3553-1-philippe.cornu@st.com