drm/radeon: use vram/gart pinned size in radeon_do_test_moves

Gives more accurate count and prevents failures when we can't
allocate memory for the tests.

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