drm/radeon: default to 2048 MB GART size on SI+

Newer ASICs have more VRAM on average and allocating more GART as
well can have advantages. Also see commit edcd26e8

.

Ideally, we should scale GART size based on actual VRAM size, but
that requires significant restructuring of initialization.

v2: extract small helper, apply to error paths

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Grigori Goronzy <greg@chown.ath.cx>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>