drm/i915: program drain latency regs on ValleyView

This patch adds support for programming drain latency registers of Pondicherry
memory arbiter of Valleyview.

v2: clarify function names (Daniel)
    fix summary typo (Daniel)
v3: add parens (Ben)
    make drain function return bool (Ben)

Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Gajanan Bhat <gajanan.bhat@intel.com>
Reviewed-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Reviewed-by: Jesse Barnes <jesse.barnes@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>