drm/amdgpu: Align amdgpu_gtt_mgr entries to TLB size on Tahiti (v2)

drm/amdgpu: Align amdgpu_gtt_mgr entries to TLB size on Tahiti (v2)

The TLB is organized in groups of 8 entries, each one is 4K.
On Tahiti, the HW requires these GART entries to be 32K-aligned.

This fixes a VCE 1 firmware validation failure that can happen
after suspend/resume since we use amdgpu_gtt_mgr for VCE 1.

v2:
- Change variable declaration order
- Add comment about "V bit HW bug"

Fixes: 698fa62f56aa ("drm/amdgpu: Add helper to alloc GART entries")
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>