drm/amd/display: Removed unused assignments and variables

[WHAT]
A number of values are assigned to variables but the stored values are
not used afterwards.

[HOW]
The assignments are removed. If the variables are not used, they are
removed as well.

This fixes 9 UNUSED_VALUE issues reported by Coverity.

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