omap_hwc: Adds a flag to hint omaplfb a blit FB is needed

When HWC generates blits a DSS pipe is configured and assigned
for the blit framebuffer, this patch adds a flag to give a
hint to omaplfb to use such pipe when it is present.

This helps in situations where there are not blits but blit
buffers to synchronize with and a blit buffer must be used.
In such case the rgz_items and num_ovls from dsscomp where not
enough data to recognize this scenario.

Change-Id: I70542b5d4a865b783277a46afe5e030d17b90f8f
Signed-off-by: Gustavo Diaz Prado <a0273371@ti.com>