glamor: Replace glamor_get/put_context() with just glamor_make_current().

glamor: Replace glamor_get/put_context() with just glamor_make_current().

Now that we have the DIX global state for the current context, we
don't need to track nesting to try to reduce MakeCurrent overhead.

v2: Fix a mistaken replacement of a put_context with make_current in
    glamor_fill_spans_gl() (caught by keithp).

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com> (v1)
Reviewed-by: Adam Jackson <ajax@redhat.com> (v1)