nouveau: Enable extensions by just setting the flags

nouveau: 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_minmax, and
EXT_blend_subtract are explicitly added to the list.

EXT_blend_logic_op is removed from the list of extensions because
blend factors and separate blend equations are not handled correctly.

Cc: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Cc: Viktor Novotný <noviktor@seznam.cz>