lima: allocate separate bo to store varyings

lima: allocate separate bo to store varyings

The current strategy using the suballocator with fixed size doesn't
scale and causes some programs with large number of vertices (like some
glmark2 scenes) to crash.
Change it to dynamically allocate a separate bo to accomodate for
arbitrary number of vertices.
This also fixes the buffer read/write flags for gp.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Reviewed-by: Andreas Baierl <ichgeh@imkreisrum.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2445>