drm/amdgpu: used cached gca values for vi_read_register (v2)

Using the cached values has less latency for bare metal
and SR-IOV, and prevents reading back bogus values if the
engine is powergated.

v2: fix typo in tile idx calculation

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