shader: on GLES most image formats always require the specification of ro/wo

Closes: #16

Fixes:
  dEQP-GLES31.functional.image_load_store.*.format_reinterpret.*
  dEQP-GLES31.functional.image_load_store.*.image_size.writeonly_*
  dEQP-GLES31.functional.image_load_store.*.load_store.*
  dEQP-GLES31.functional.image_load_store.*.qualifiers.restrict
  dEQP-GLES31.functional.image_load_store.*.store.*

v2: remove superfluous check (Erik)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>