hwc: rgz: Fix using an invalid array index when there are no blits

When the regionizer decides there are no blits as a result of a given
hwc geometry a -1 array index was being used to set the last blit
async, this needs to be avoided if there are no blits.

Change-Id: I3ce2f60a46096559c2ba308cf1d617a79731751e
Signed-off-by: Gustavo Diaz Prado <a0273371@ti.com>
(cherry picked from commit 8d486cd705ffc33d6048880189e16fed387338fd)
1 file changed