libagl: Allow use of copybit to accelerate 2D rendering

This appears to fix the 2D performance issues on msm7225 and msm7227,
no more stutter in animations or transitions
Enable with TARGET_LIBAGL_USE_GRALLOC_COPYBITS := true

Change-Id: If1e06dc2b7836e370053d1c664fe5192619e3015