i965: Refactor Gen4-6 SURFACE_STATE setup for buffer surfaces.

i965: Refactor Gen4-6 SURFACE_STATE setup for buffer surfaces.

This was an embarassingly large amount of copy and pasted code,
and it wasn't particularly simple code either.  By factoring it out
into a helper function, we consolidate the complexity.

v2: Properly NULL-check bo.  Caught by Eric Anholt.
v3: Do the subtraction by 1 in gen7_emit_buffer_surface_state, rather
    than making callers do it.  This makes the buffer_size parameter
    the actual size of the buffer.  Suggested by Paul Berry.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>