t_dd_dmatmp: Pull out common 'count -= count & 3' code

t_dd_dmatmp: Pull out common 'count -= count & 3' code

This was missing in the HAVE_TRIANGLES path, and that could cause
incorrect rendering.

No piglit regressions on i915 (G33) or radeon (Radeon 7500).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38109
Reviewed-by: Brian Paul <brianp@vmware.com>
Cc: Marius Predut <marius.predut@intel.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit c0b3b2f7603eab210acdb2e654e5411fe912ca34)