drm/msm/mdp5: Add check for kzalloc

As kzalloc may fail and return NULL pointer,
it should be better to check the return value
in order to avoid the NULL pointer dereference.

Fixes: 1cff7440a86e ("drm/msm: Convert to using __drm_atomic_helper_crtc_reset() for reset.")
Signed-off-by: Jiasheng Jiang <[e-mail]>
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Patchwork: https://patchwork.freedesktop.org/patch/514154/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Baryshkov <[e-mail]>

drivers/gpu/drm/msm/disp/mdp5/mdp5_crtc.c

···
 	if (crtc->state)
 		mdp5_crtc_destroy_state(crtc, crtc->state);
 
-	__drm_atomic_helper_crtc_reset(crtc, &mdp5_cstate->base);
+	if (mdp5_cstate)
+		__drm_atomic_helper_crtc_reset(crtc, &mdp5_cstate->base);
+	else
+		__drm_atomic_helper_crtc_reset(crtc, NULL);
 }
 
 static const struct drm_crtc_funcs mdp5_crtc_no_lm_cursor_funcs = {