drm/amd/display/dc/dce/dce_transform: Remove 3 unused/legacy variables

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_transform.c: In function ‘dce60_transform_set_scaler’:
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_transform.c:496:7: warning: variable ‘filter_updated’ set but not used [-Wunused-but-set-variable]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_transform.c: In function ‘dce60_transform_set_pixel_storage_depth’:
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_transform.c:1040:19: warning: variable ‘expan_mode’ set but not used [-Wunused-but-set-variable]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_transform.c:1040:6: warning: variable ‘pixel_depth’ set but not used [-Wunused-but-set-variable]

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Mauro Rossi <issor.oruam@gmail.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>