drm/amd/display: Check null pointer before dereferencing se

[WHAT & HOW]
se is null checked previously in the same function, indicating
it might be null; therefore, it must be checked when used again.

This fixes 1 FORWARD_NULL issue reported by Coverity.

Acked-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Alex Hung

drivers/gpu/drm/amd/display/dc/core/dc.c

@@ -1767,7 +1767,7 @@ bool dc_validate_boot_timing(const struct dc *dc,
 		if (crtc_timing->pix_clk_100hz != pix_clk_100hz)
 			return false;
 
-		if (!se->funcs->dp_get_pixel_format)
+		if (!se || !se->funcs->dp_get_pixel_format)
 			return false;
 
 		if (!se->funcs->dp_get_pixel_format(