drm/amdgpu: rename static functions in amdgpu_display.c

Add display to the name for consistency.

Signed-off-by: Samuel Li <Samuel.Li@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>