drm/amd/display: reduce scope for local var

[Why]
No reason to have uninitialized var at top of function

[How]
Move it to minimum scope where it's actuallty used

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>