egl: Add support for DRM_FORMAT_R8, RG88, and GR88

egl: Add support for DRM_FORMAT_R8, RG88, and GR88

The Kodi/XBMC developers want to transcode NV12 to RGB with OpenGL shaders,
importing the two source planes through EGL_EXT_image_dma_buf_import. That
requires importing the Y plane as an R8 EGLImage and the UV plane as either an
RG88 or GR88 EGLImage.

This patch teaches the driver-independent part of EGL about the new
formats. Real driver support is left for follow-up patches.

The new formats landed in airlied's kernel branch 'drm-next' on July 24.

Tested-by: Peter Frühberger <peter.fruehberger@gmail.com>
Signed-off-by: Chad Versace <chad.versace@intel.com>