drm/amdgpu: keep vga memory on MacBooks with switchable graphics

drm/amdgpu: keep vga memory on MacBooks with switchable graphics

On Intel MacBookPros with switchable graphics, when the iGPU
is enabled, the address of VRAM gets put at 0 in the dGPU's
virtual address space.  This is non-standard and seems to cause
issues with the cursor if it ends up at 0.  We have the framework
to reserve memory at 0 in the address space, so enable it here if
the vram start address is 0.

Reviewed-and-tested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4302
Cc: stable@vger.kernel.org
Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>