drm/msm/dpu: update bandwidth threshold check

Maximum allowed bandwidth  has no dependency on the type
of panel used. Hence, cleanup the code to use max_bw_high
as the threshold value for bandwidth checks.

Update the maximum allowed bandwidth as 6.8Gbps for
SC7180 target.

Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>