i965: Add a RGBX->RGBA fallback for glEGLImageTextureTarget2D()

i965: Add a RGBX->RGBA fallback for glEGLImageTextureTarget2D()

This enables support for importing RGBX8888 EGLImage textures on
Skylake.

Chrome OS needs support for RGBX8888 EGLImage textures because because
the Android framework produces HAL_PIXEL_FORMAT_RGBX8888 winsys
surfaces, which the Chrome OS compositor consumes as dma_bufs.  On
hardware for which RGBX is unsupported or disabled, normally core Mesa
provides the RGBX->RGBA fallback during glTexStorage.  But the DRIimage
code bypasses core Mesa, so we must do the fallback in i965.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>