drm/amd/display: Add missing static

After enable DRM_AMDGPU_WERROR, clang highlight multiple functions that
need to have `static`, and this commit address those issues and also
improve the indents.

Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>