nv30: only bail on color/depth bpp mismatch when surfaces are swizzled

nv30: only bail on color/depth bpp mismatch when surfaces are swizzled

The actual restriction is a little weaker than I originally thought. See
https://bugs.freedesktop.org/show_bug.cgi?id=92306#c17 for the
suggestion. This also explain why things weren't *always* failing
before, only sometimes. We will allocate a non-swizzled depth buffer for
NPOT winsys buffer sizes, which they almost always are.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org