drm_hwcomposer: Support relaxed requirements for the most bottom plane

1. Some of DRM/KMS drivers has no planes with formats which Android does require,
   but in some cases it's possible to modify the format of the buffer,
   making such planes usable without any drawbacks.

2. Another use-case is blend mode support. Android does require premultiplied
   blending mode support for all planes, but such requirement can be made
   optional for the most bottom plane without any drawbacks.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
2 jobs for most-bottom-plane-special-handling in 11 minutes and 49 seconds (queued for 23 seconds)