drm/radeon: default to 1024M gart size on rv770+

Newer asics have a lot of vram so it's less of an
issue to waste a little more space for the gart
page table.  This gives us some additional gart space
before having to migrate to non-gart system ram
for games, etc. where we use up most of vram.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>