drm/amdgpu/gmc: steal the appropriate amount of vram for fw hand-over (v3)

Steal 9 MB for vga emulation and fb if vga is enabled, otherwise,
steal enough to cover the current display size as set by the vbios.

If no memory is used (e.g., secondary or headless card), skip
stolen memory reserve.

v2: skip reservation if vram is limited, address Christian's comments
v3: squash in fix from Harry

Reviewed-and-Tested-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com> (v2)
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>