drm/amd/display: Add Null check for DPP resource

drm/amd/display: Add Null check for DPP resource

[what and why]
Check whether dpp resource pointer is null in advance and return early
if so.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Martin Leung <martin.leung@amd.com>
Signed-off-by: Gabe Teeger <gabe.teeger@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>