i965: Implement EGL_KHR_mutable_render_buffer

i965: Implement EGL_KHR_mutable_render_buffer

Testing:
  - Manually tested a low-latency handwriting demo that toggles
    EGL_RENDER_BUFFER. Toggling changed the display latency as expected.
    Used Android on Chrome OS, Kabylake GT2.
  - No change in dEQP-EGL.functional.* on Fedora 27, Wayland, Skylake
    GT2.  Used deqp at tag android-p-preview-5.
  - No regressions in dEQP-EGL.functional.*, ran on Android on Chrome
    OS, Kabylake GT2. Some dEQP-EGL.functional.mutable_render_buffer.*
    test change from NotSupported to Pass.

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