drm/i915: Set persistent-mode for ILK/SNB framebuffer compression

Persistent mode is intended for use with front-buffer rendering, such as
X, where it is necessary to detect writes to the scanout either by the
GPU or through the CPU's fence, and recompress the dirty regions on the
fly. (By comparison to the back-buffer rendering, the scanout is always
recompressed after a page-flip.)

References: https://bugs.freedesktop.org/show_bug.cgi?id=33487
References: https://bugs.freedesktop.org/show_bug.cgi?id=31742


Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>