nv50: add support for texelFetch'ing MS textures, ARB_texture_multisample

nv50: add support for texelFetch'ing MS textures, ARB_texture_multisample

Creates two areas in the AUX constbuf:
 - Sample offsets for MS textures
 - Per-texture MS settings

When executing a texelFetch with a MS sampler, looks up that texture's
settings and adjusts the parameters given to the texfetch instruction.

With this change, all the ARB_texture_multisample piglits pass, so turn
on PIPE_CAP_TEXTURE_MULTISAMPLE.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>