i965: Pull out INTEL_DEBUG handling into new intel_debug.[ch] files.

i965: Pull out INTEL_DEBUG handling into new intel_debug.[ch] files.

Now that there isn't an intel_context structure, the split between
brw_context.[ch] and intel_context.[ch] is rather awkward and arbitrary.
Removing intel_context.[ch] seems desirable, but not everything really
belongs in brw_context.[ch], either.

Moving INTEL_DEBUG handling into separate intel_debug.[ch] files should
make them relatively easy to find.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>