drm/amd/display: Remove unused local variables

Remove local variables that were just set but were never used. This
decrease the number of -Wunused-but-set-variable warnings.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>