st/mesa: fix trimming of GL_QUAD_STRIP

st/mesa: fix trimming of GL_QUAD_STRIP

We sometimes convert GL_QUAD_STRIP prims into GL_TRIANGLE_STRIP, but
that changes the results of the u_trim_pipe_prim() call.  We need to
pass the original primitive type to the trim function.

Note that OpenGL's GL_x prim type values match Gallium's PIPE_PRIM_x values.

Fixes a failure in the new piglit degenerate-prims test.

Note: This is a candidate for the stable branches.

Reviewed-by: José Fonseca <jfonseca@vmware.com>