drm/radeon: use vzalloc for gart pages

When allocating more than 2GB of GART the array of pages
gets to big for kzalloc, use vzalloc instead.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>