drm/amd/amdgpu: remove defined but not used 'crtc_offsets'

Fix the following gcc warning:

drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c:65:18: warning: ‘crtc_offsets’
defined but not used [-Wunused-const-variable=]
 static const u32 crtc_offsets[6] =
                  ^~~~~~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>