drm/amdgpu: fix the overflowed constant warning for RREG32_SOC15()

To fix potential overflowed constant warning reported by Coverity,
modify the variables to uint32_t.

Signed-off-by: Bob Zhou <bob.zhou@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>