drm/msm/gpu: add support for ocmem interconnect path

Some A3xx and all A4xx Adreno GPUs do not have GMEM inside the GPU core
and must use the On Chip MEMory (OCMEM) in order to be functional.
There's a separate interconnect path that needs to be setup to OCMEM.
Add support for this second path to the GPU core.

In the downstream MSM 3.4 sources, the two interconnect paths for the
GPU are between:

  - MSM_BUS_MASTER_GRAPHICS_3D and MSM_BUS_SLAVE_EBI_CH0
  - MSM_BUS_MASTER_V_OCMEM_GFX3D and MSM_BUS_SLAVE_OCMEM

Signed-off-by: Brian Masney <masneyb@onstation.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>