i915: Drop broken front_buffer_reading/drawing optimization

i915: Drop broken front_buffer_reading/drawing optimization

Bring the following commit over to i915:
 commit ec542d74578bbef6b55125dd6aba1dc7f5079e65
 Author: Eric Anholt <eric@anholt.net>
 Date:   Mon Mar 3 10:43:10 2014 -0800

    i965: Drop broken front_buffer_reading/drawing optimization.

Not sure if it might fix anything, but since the i965 and i915 used to
share a bunch of that code, it would seem reasonable the same problems
could be present in the i915 code still, and the i965 approach is well
tested by now so bringing it over seems fairly safe.

No piglit regressions on 855.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>