drm/msm/a6xx: Resolve the meaning of UBWC_MODE

drm/msm/a6xx: Resolve the meaning of UBWC_MODE

This bit is set iff the UBWC version is 1.0. That notably does not
include QCM2290's "no UBWC".

This commit is intentionally cross-subsystem to ease review, as the
patchset is intended to be merged together, with a maintainer
consensus.

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/660971/
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>