svga: do primitive trimming in translate_indices()

svga: do primitive trimming in translate_indices()

The index translation code expects the number of indexes to be
consistent with the primitive type (ex: a multiple of 3 for
PIPE_PRIM_TRIANGLES).  If it's not, we can write out of bounds
in the destination buffer.

Fixes failed assertions in the pipebuffer debug code found with
Piglit primitive-restart-draw-mode test.

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: José Fonseca <jfonseca@vmware.com>