gallium: Add a helper for implementing PIPE_CAP_* default values.

gallium: Add a helper for implementing PIPE_CAP_* default values.

One of the pains of implementing a gallium driver is filling in a million
pipe caps you don't know about yet when you're just starting out.  One of
the pains of working on gallium is copy-and-pasting your new PIPE_CAP into
each driver.  We can fix both of these by having each driver call into the
default helper from their default case, so that both sides can ignore each
other until they need to.

v2: fix i915g build, revert swr change to avoid breaking scons build
    (https://travis-ci.org/anholt/mesa/jobs/419739857)
v3: Rebase on 3 new gallium caps.

Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)
Cc: Bruce Cherniak <bruce.cherniak@intel.com>
Cc: George Kyriazis <george.kyriazis@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>