gbm: Add a native intel backend
gbm_drm: Keep a reference to drm native objects

Add bo and bufmgr fields to gbm_drm_bo and gbm_drm_device respectively.
dri, i965: Add an extension for sharing the drm bufmgr

If the fd used by the DRI driver is going to be shared with something
else, like gbm, this lets the loader pass the bufmgr struct so it is
shared too.
dri, i965: Add entry point for creating image from native handle

The native handle is a pointer to drm_intel_bo for the i965 driver.
egl/drm: Add support for native gbm backends
gbm: Add entry points for mapping and unmapping bos

Add gbm_bo_map() and gbm_bo_unmap(). This lets a user access the
contents of a bo using the CPU.
gbm: Add support for exporting a bo to a dma_buf