nv50: use 2d blit when src/dst have same number of samples

The 2D engine should be usable in more cases, but this fixes MS blits
between textures with the same MS settings. Otherwise a single sample is
selected to be the target texel value.

This allows other tests to work that render to a RB and then blit that
to a texture for input into a shader that uses sampler2DMS to verify it.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nvc0: fix centering of eng2d blit when downsampling

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