drm/msm: mdp4: Fix drm_framebuffer dereference crash

mdp4_get_frame_format() can dereference a drm_framebuffer when it's NULL.
Call it in mdp4_plane_mode_set only when we know fb is non-NULL.

Signed-off-by: Archit Taneja <[e-mail]>
Signed-off-by: Rob Clark <[e-mail]>

drivers/gpu/drm/msm/mdp/mdp4/mdp4_plane.c

···
 	uint32_t op_mode = 0;
 	uint32_t phasex_step = MDP4_VG_PHASE_STEP_DEFAULT;
 	uint32_t phasey_step = MDP4_VG_PHASE_STEP_DEFAULT;
-	enum mdp4_frame_format frame_type = mdp4_get_frame_format(fb);
+	enum mdp4_frame_format frame_type;
 
 	if (!(crtc && fb)) {
 		DBG("%s: disabled!", mdp4_plane->name);
 		return 0;
 	}
+
+	frame_type = mdp4_get_frame_format(fb);
 
 	/* src values are in Q16 fixed point, convert to integer: */
 	src_x = src_x >> 16;