msm: mdss: propagate pipe priority to user-space

Priority is returned by the driver when overlay is set for the first
time. It indicates the priority of the underlying pipe serving the
overlay. This priority can be used by user-space in source split when
pipes are re-used and shuffled around to reduce fall-backs related
priority restrictions.

Change-Id: Ibfda0280279e7057a1d3494f4e5f68962b26ef4a
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>