i965: Move contents of brw_tex.c into intel_tex_validate.c.

i965: Move contents of brw_tex.c into intel_tex_validate.c.

brw_tex.c is a tiny file containing a single function.  It's closely
tied to the validation logic in intel_tex_validate.c, so it makes sense
to put both in the same file.

While we're at it, update the function to our modern style.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>