drm/amdgpu/vce1: Ensure VCPU BO is in lower 32-bit address space (v3)

drm/amdgpu/vce1: Ensure VCPU BO is in lower 32-bit address space (v3)

Based on research and ideas by Alexandre and Christian.

VCE1 actually executes its code from the VCPU BO.
Due to various hardware limitations, the VCE1 requires
the VCPU BO to be in the low 32 bit address range.
However, VRAM is typically mapped at the high address range,
which means the VCPU can't access VRAM through the FB aperture.

To solve this, we write a few page table entries to
map the VCPU BO in the GART address range. And we make sure
that the GART is located at the low address range.
That way the VCE1 can access the VCPU BO.

v2:
- Adjust to v2 of the GART helper commit.
- Add empty line to multi-line comment.

v3:
- Instead of relying on gmc_v6 to set the GART space before GTT,
  add a new function amdgpu_vce_required_gart_pages() which is
  called from amdgpu_gtt_mgr_init() directly.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Co-developed-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Signed-off-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>