radeon: Enable extensions by just setting the flags

radeon: Enable extensions by just setting the flags

Core Mesa already does the dispatch offset remapping for every
function that could possibly ever be supported.  There's no need to
continue using that cruft in the driver.

Since the call to _mesa_enable_imaging_extensions (via
driInitExtensions) is removed, EXT_blend_color, EXT_blend_logic_op,
and EXT_blend_minmax are no longer advertised.  These all resulted in
software fallbacks, so their loss will not be mourned.
EXT_blend_subtract is, however, explicitly added to the list.
GL_FUNC_SUBTRACT is fully accelerated, but GL_FUNC_REVERSE_SUBTRACT
(still) results in a software fallback.

Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Dave Airlie <airlied@redhat.com>