nvc0: disable BGRA8 images on Fermi

nvc0: disable BGRA8 images on Fermi

BGRA8 image stores on Fermi don't work, which results in breaking
PBO downloads, such that they always return 0x0. Discovered this
through a glamor bug, and confirmed it does indeed break a good number
of piglit tests such as spec/arb_pixel_buffer_object/pbo-read-argb8888

Fixes: 8e7893eb53213 ("nvc0: add support for BGRA8 images")
Signed-off-by: Lyude <lyude@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org