egl/dri: flesh out and use dri2_create_drawable()

egl/dri: flesh out and use dri2_create_drawable()

Wrap the loader->createNewDrawable() dance into a helper and use it
throughout the codebase.

This addresses a cases like surfaceless (SL) on swrast (SL on kms_swrast
is fine) where we'd attempt using the wrong driver and crash out.

v2: fixup quirky GBM (Mathias)
v3: fixup GBM for real (Marek)

Cc: mesa-stable@lists.freedesktop.org
Cc: Mathias Fröhlich <Mathias.Froehlich@web.de>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de> (v1)
Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)
Signed-off-by: Emil Velikov <emil.velikov@collabora.com> (v2)
Signed-off-by: Marek Olšák <marek.olsak@amd.com> (v2)
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>