drm/amdgpu: remove set but not used variable 'adev'

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c:1221:24: warning: variable adev set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c:488:24: warning: variable adev set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c:547:24: warning: variable adev set but not used [-Wunused-but-set-variable]

It is never used, so can removed it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>