drm/radeon: Restore GART table contents after pinning it in VRAM v3

drm/radeon: Restore GART table contents after pinning it in VRAM v3

The GART table BO has to be moved out of VRAM for suspend/resume. Any
updates to the GART table during that time were silently dropped without
this change. This caused GPU lockups on resume in some cases, see the bug
reports referenced below.

This might also make GPU reset more robust in some cases, as we no longer
rely on the GART table in VRAM being preserved across the GPU
lockup/reset.

v2: Add logic to radeon_gart_table_vram_pin directly instead of
    reinstating radeon_gart_restore
v3: Move code after assignment of rdev->gart.table_addr so that the GART
    TLB flush can work as intended, add code comment explaining why we're
    doing this

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85204
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86267
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>