drm/amdgpu/gmc10: adjust gart size for parts that support S/G display

For GMC 10 parts which support scatter/gather display (display
from system memory), we should allocate a larger gart size
to better handler larger displays.  This mirrors what we already
do for GMC 9 parts.

v2: fix typo (Alex)

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>