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>