hwc: Fix alpha downscale check for mdp composition

Do not allow mdp composition when any layer format has alpha channel
and needs scaling as the VG pipes are not supporting alpha downscaling.

In few scenarios, at the beginning/ending of animations Surface Flinger
passes few frames with flag HWC_BLENDING_NONE though it has Alpha.
When such layer passed to VG piple and if needs scaling , it results in
flicker. Hence, just checking for HWC_BLENDING_NONE is not enough.

Change-Id: I70fdfe4547df6fe7b2771ed8042f43aa4e07a71d
CRs-fixed: 462508
libcopybit: Remove usage of C2D_NO_BILINEAR_BIT

Remove usage of C2D_NO_BILINEAR_BIT in the config mask.
This enables using the BILINEAR filter for scaling and
improves the image quality

Change-Id: Ic9b61fe3ed5b16e63971fe542928640a13d92764
CRs-fixed: 453925
hwc: Add support for clearing with c2d

When blending translucent layers or transitioning with c2d,
sometimes the frame is blended with stale framebuffer content.
Clear the frame with C2D fill surface before drawing.

CRs-fixed: 462461
Change-Id: I1dca477fe88d77f4f0fa4846b155404f348e911d

Conflicts:
	libcopybit/copybit_c2d.cpp