drm/amd/display: Validate function returns

[WHAT & HOW]
Function return values must be checked before data can be used
in subsequent functions.

This fixes 4 CHECKED_RETURN issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@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>