drm/amd/display: Remove unused local variables and function

Remove a couple of local variables that are only set but never used,
also remove an static utility function that is never used in consequence
of the variable removal.

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>