some clean up to intel-gtt.c

In commit e517a5e9 the call to
map_page_into_agp() got removed from intel_i830_setup_flush(), but the
counterpart call from intel_i830_fini_flush() to unmap_page_from_agp()
was left in place.

Additionally, the page allocated here never gets its physical address
used for sending to hardware, so there's no need to allocate it with
GFP_DMA32. Nor is __GFP_ZERO really necessary, as the page is used
only to store data to force flushing of some internal processor state.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>