Specifically, implement the extension DRI_MutableRenderBufferLoader.
However, the loader enables EGL_KHR_mutable_render_buffer only if the
DRI driver implements its half of the extension,
DRI_MutableRenderBufferDriver.

Testing:
  - No change in dEQP-EGL.functional.* on Fedora 27, Wayland, Skylake
    GT2.  Used deqp at tag android-p-preview-5.
  - No change in dEQP-EGL.functional.*, ran on Android on Chrome OS,
    Kabylake GT2.
  - Manually inspected Android apps on same Chrome OS device.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>