drm/msm/sde: enforce pipe priority restrictions

Need to consider hardware pipe priority when validating an
atomic commit. Hardware requires certain source pipes to be
left-most compared to other source pipes when in source split
configurations.

CRs-Fixed: 2005394
Change-Id: I4a19fbe9f78db8e557720ef7973034fdbd2f8a58
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>