Refactor code that converts between gl_vert_result and gl_frag_attrib.

Refactor code that converts between gl_vert_result and gl_frag_attrib.

Previously, this conversion was duplicated in several places in the
i965 driver.  This patch moves it to a common location in mtypes.h,
near the declaration of gl_vert_result and gl_frag_attrib.

I've also added comments to remind us that we may need to revisit the
conversion code when adding elements to gl_vert_result and
gl_frag_attrib.

Reviewed-by: Eric Anholt <eric@anholt.net>