drm/ttm: DMA map/unmap consecutive pages as a whole v2

Instead of mapping them bit by bit map/unmap all consecutive
pages as in one call.

v2: test for consecutive pages instead of using compound page order.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>