msm: display: Add pipe type selection in overlay request

Allow user to specify the type when requesting an overlay pipe.
If field is set the pipe type indication in the flags will be
ignored.
Change-Id: Iee51d24b96c7a2a987dc70a439342e6548dfc8fa
Signed-off-by: Vineet Bajaj <vbajaj@codeaurora.org>