gallium/util: allow non average MS resolution in blit

This allows blitter to not perform averaging when blitting a multisampled color buffer.

V3D has a blit path where it basically re-interprets the stencil buffer as a color buffer, and uses the blitter utility to do the blit. But the resolve in the blitter do a linear or bilinear average sampling, which is correct for color buffers, but it is incorrect for stencil buffers. So we need to instruct the blitter to not do an average in this case.

This can be observed in the piglit's test ext_framebuffer_multisample-unaligned-blit 2 stencil downsample -auto -fbo: in V3D the triangles borders are averaged, when they shouldn't.