drm/amd/display: Make variables declaration inside ifdef guard

Make variables declaration inside ifdef guard, as they are only used
inside the same ifdef guard. This remove some of the
-Wunused-but-set-variable warning.

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