drm/radeon/kms: fix gtt MC base alignment on rs4xx/rs690/rs740 asics

The asics in question have the following requirements with regard to
their gart setups:

1. The GART aperture size has to be in the form of 2^X bytes, where X is from 25 to 31
2. The GART aperture MC base has to be aligned to a boundary equal to the size of the
aperture.
3. The GART page table has to be aligned to the boundary equal to the size of the table.
4. The GART page table size is: table_entry_size * (aperture_size / page_size)
5. The GART page table has to be allocated in non-paged, non-cached, contiguous system
memory.

This patch takes care 2.  The rest should already be handled properly.

This fixes a regression noticed by: Torsten Kaiser <just.for.lkml@googlemail.com>

Tested-by: Torsten Kaiser <just.for.lkml@googlemail.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>