drm/vkms: fix variable dereferenced before check warning

Smatch warns:

drivers/gpu/drm/vkms/vkms_plane.c:110 vkms_plane_atomic_update() warn:
 variable dereferenced before check 'fb' (see line 108)

Fix the warning by moving the dereference after the NULL check.

Fixes: 8ba16485 ("drm: vkms: Refactor the plane composer to accept new formats")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220908105623.72777-1-harshit.m.mogalapalli@oracle.com