drm/msm/a6xx: Use the DMA API for GMU memory objects

The GMU has very few memory allocations and uses a flat memory space so
there is no good reason to go out of our way to bypass the DMA APIs which
were basically designed for this exact scenario.

v7: Check return value of dma_set_mask_and_coherent
v4: Use dma_alloc_wc()
v3: Set the dma mask correctly and use dma_addr_t for the iova type
v2: Pass force_dma false to of_dma_configure to require that the DMA
region be set up and return error from of_dma_configure to fail probe.

Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>