New now ask Glamor to use EGL_MESA_query_driver to obtain the DRI driver
name; if successful, we use that as the DRI driver name.  Following the
existing dri2.c logic, we also use the same name for the VDPAU driver,
except for i965 (and now iris), where we switch to the "va_gl" fallback.

This allows us to bypass the PCI ID lists in xserver and centralize the
driver selection mechanism inside Mesa.  The hope is that we no longer
have to update these lists for any future hardware.