drm/msm/sde: add rectangle utility functions to sde

Add rectangle intersection and other utility functions to the
sde kms utilities.

Change-Id: I10a1f60480df10a93541990b3efa347264c6432f
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>