gallium/u_blitter: fix is_blit_generic_supported() stencil checking

gallium/u_blitter: fix is_blit_generic_supported() stencil checking

Don't check if there's sampler support for stencil if we're not
going to actually blit/copy stencil values.  Fixes the case where
we mistakenly said we can't support a blit of depth values from
S8Z24 to X8Z24.

Also, rename the is_stencil variable to dst_has_stencil to improve
readability.

NOTE: This is a candidate for the stable branches.

Reviewed-by: Marek Olšák <maraeo@gmail.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>