winsys/radeon: align BO size to page size

winsys/radeon: align BO size to page size

This is the basic granularity for BO allocations. The alignment also
helps with BO reuse by the cached bufmgr.

This results in a huge 45% speedup in Metro 2033 Redux on my test
system. The game relies on buffer orphaning with very small buffers
(hundreds of bytes in size) and that did not work efficiently
before. This change may also affect other applications and games.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>