drm_hwcomposer: Extract ConvertBoInfo() from ImportBuffer() method

DEPENDS on !78 (merged)

To check either BO could be imported on validation stage, we need to
get more information regarding buffer (format, size, etc.)

This extraction also allows us to make ImportBuffer() and CanImport()
methods generic for all platforms.

In case BO can't be imported due to already existant checks in ImportBuffer(),
validator only marks single layer to be elaborated by GPU instead of whole
composition.

Signed-off-by: Roman Stratiienko roman.stratiienko@globallogic.com