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

drivers/gpu/drm/vkms/vkms_plane.c

@@ -105,11 +105,12 @@ static void vkms_plane_atomic_update(struct drm_plane *plane,
 	struct drm_shadow_plane_state *shadow_plane_state;
 	struct drm_framebuffer *fb = new_state->fb;
 	struct vkms_frame_info *frame_info;
-	u32 fmt = fb->format->format;
+	u32 fmt;
 
 	if (!new_state->crtc || !fb)
 		return;
 
+	fmt = fb->format->format;
 	vkms_plane_state = to_vkms_plane_state(new_state);
 	shadow_plane_state = &vkms_plane_state->base;