libhwcomposer: Don't reset cached layer handle on MDP comp

Layer handle values will always be stored irrespective of the
composition type. Layer caching will choose a layer for FB
composition if it's handle has not changed.

Change-Id: Ied42d6ecafe48a9240aacc2d172b7d81b0d42a5d
Don't force GPU composite for older devices

Older devices (8960) don't take this very well in programs like
Netflix. Modeled on 94b8632b85e2b6588a1b3260d1f4e48462501d4b

Change-Id: Ie716bbfb6f6560d8f9f52b7b72ca0b04d1bad6b8
liboverlay: do not return error if fb num >= 2 doesn't exist (writeback).

Some devices doesn't have writeback support, but needs HDMI (fb1)
This will allow them not to break HDMI support.
This is a cleaner implementation as it won't change handling on
8660 devices having writeback (if those exists).

Change-Id: I6c09759dfa4f2b1734f85012cab4eb4f350820ea
hwc: poll for vsync event

poll on the vsync file descriptor. This prepares the ground for
multiple vsync updates without adding additional threads.

Change-Id: I30ef0fb34816d438501475cd3e4967775fcb1d19