st/mesa: rewrite the primitive restart fallback code

st/mesa: rewrite the primitive restart fallback code

Previously we were mapping/unmapping the index buffer each time we
found the restart index in the buffer.  This is bad when the restart
index is frequently used.  Now just map the index buffer once, scan
it to produce a list of sub-primitives, unmap the buffer, then draw
the sub-primitives.

Also, clean up the logic of testing for indexed primitives and calling
handle_fallback_primitive_restart().  Don't call it for non-indexed
primitives.

v2: per Jose, only map the relevant part of the index buffer with
pipe_buffer_map_range()

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