drm/amd/display: Remove the redundant NULL check

Static analysis shows that pointer "timing" cannot be NULL because it
points to the object "struct dc_crtc_timing".

Remove the extra NULL check. It is meaningless and harms the readability
of the code.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Andrey Vatoropin <a.vatoropin@crpt.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/resource/dcn30/dcn30_resource.c

@@ -1891,8 +1891,6 @@ static int get_refresh_rate(struct dc_state *context)
 
 	/* check if refresh rate at least 120hz */
 	timing = &context->streams[0]->timing;
-	if (timing == NULL)
-		return 0;
 
 	h_v_total = timing->h_total * timing->v_total;
 	if (h_v_total == 0)