drm/radeon/r6xx: fix DMA engine for ttm bo transfers

count must be a multiple of 2. Fixes crashes on
R6xx chips reported by a number of people.

Cc: Borislav Petkov <bp@alien8.de>
Cc: Markus Trippelsdorf <markus@trippelsdorf.de>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Tested-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>