drm/stm: ltdc: fix late dereference check

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

drm/stm: ltdc: fix late dereference check

In ltdc_crtc_set_crc_source(), struct drm_crtc was dereferenced in a

container_of() before the pointer check. This could cause a kernel panic.

Fix this smatch warning:

drivers/gpu/drm/stm/ltdc.c:1124 ltdc_crtc_set_crc_source() warn: variable dereferenced before check 'crtc' (see line 1119)