mesa: implement GL_AMD_pinned_memory

mesa: implement GL_AMD_pinned_memory

It's not possible to query the current buffer binding, because the extension
doesn't define GL_..._BUFFER__BINDING_AMD.

Drivers should check the target parameter of Drivers.BufferData. If it's
equal to GL_EXTERNAL_VIRTUAL_MEMORY_BUFFER_AMD, the memory should be pinned.
That's all there is to it.

A piglit test is on the piglit mailing list.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>