drm/msm/a6xx: Get a handle to the common UBWC config

drm/msm/a6xx: Get a handle to the common UBWC config

Start the great despaghettification by getting a pointer to the common
UBWC configuration, which houses e.g. UBWC versions that we need to
make decisions.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/660965/
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>