nvc0: add preliminary support for images

nvc0: add preliminary support for images

This implements set_shader_images() and resource invalidation for
images. As OpenGL requires at least 8 images, we are going to expose
this minimum value even if this might be raised for Kepler, but this
limit is mainly for Fermi because the hardware only accepts 8 images.

Based on original patch by Ilia Mirkin.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>