drm/amd/display: Fix null pointer issues found in emulation

[why]
fix null point issues found in emulation

Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>