drm: Remove the dma_alloc_coherent wrapper for internal usage

Internally for "consistent" maps, we create a temporary struct
drm_dma_handle in order to use our own dma_alloc_coherent wrapper then
destroy the temporary wrap. Simplify our logic by removing the temporary
wrapper!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200202171635.4039044-2-chris@chris-wilson.co.uk