drm/amdgpu/gart: Add helper to bind VRAM pages (v2)

drm/amdgpu/gart: Add helper to bind VRAM pages (v2)

Binds pages that located in VRAM to the GART page table.

Useful when a kernel BO is located in VRAM but
needs to be accessed from the GART address space,
for example to give a kernel BO a 32-bit address
when GART is placed in LOW address space.

v2:
- Refactor function to be more reusable

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>