drm/amdgpu: optimize amdgpu driver load & resume time

amdgpu_device_resume() & amdgpu_device_init() have a high
time consuming call of amdgpu_late_init() which sets the
clock_gating state of all IP blocks and is blocking.
This patch defers only this setting of clock gating state
operation to post resume of amdgpu driver but ideally before
the UI comes up or in some cases post ui as well.

With this change the resume time of amdgpu_device comes down
from 1.299s to 0.199s which further helps in reducing the overall
system resume time.

V1: made the optimization applicable during driver load as well.

TEST:(For ChromiumOS on STONEY only)
* UI comes up
* amdgpu_late_init() call gets called consistently and no errors reported.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>