drm/amd/display: Clean up NULL pointer warnings in dml2

drm/amd/display: Clean up NULL pointer warnings in dml2

This commit addresses multiple warnings by adding defensive
checks for NULL pointers before dereferencing them. The changes ensure
that pointers such as are validated, preventing potential undefined
behavior.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Gaghik Khachatrian <gaghik.khachatrian@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>