Import gbm internal header files.

Import gbm internal header files.

Prepartion to support cl_khr_gl_sharing extension. I
decide to use egl/gbm/dri2 image extension to implement
it.

The reason why we need to import gbm internal header files
is that gbm doesn't initialize the image extension by default
with x11 platform. As it doesn't know the default display and
the context when create a gbm device. We have to use its internal
structure to initialize the image extension properly after the
gbm device's creation. We are not the only one to do that way,
the EGL drm platform does the same thing.

To avoid dependencies to the whole mesa source package, I have
to copy the related header files here.

Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>
Tested-by: Homer Hsing <homer.xing@intel.com>