android: Move gralloc handle struct to libdrm

This struct is used in mesa and drm_hwcomposer.
Versions of if have been implemented in several grallocs:
drm_gralloc, gbm_gralloc, minigbm and intel-minigbm.

Other than the 1:1 move of the struct a new generic name
has been chosen and variables have had comments added to them.

Signed-off-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>