i915: Handle provoking vertex in intelFastRenderClippedPoly()

i915: Handle provoking vertex in intelFastRenderClippedPoly()

intelFastRenderClippedPoly() renders the polygon using triangles. For
polygons the provoking vertex is always the first one, and currently
this function assumes that the provoking vertex for triangles is the
last one. In case the user changed the provoking vertex convention,
the hardware may be configured to treat the first vertex of triangles
as the provoking vertex. So check the convention and emit the triangles
in the appropriate order to avoid having to change the hardware
provoking vertex convention for rendering polygons.

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