drm/i915: turbo & RC6 support for VLV v7

Uses slightly different interfaces than other platforms.

v2: track actual set freq, not requested (Rohit)
    fix debug prints in init code (Jesse)
v3: don't write sleep reg (Jesse)
    re-add RC6 wake limit write (Ben)
    fixup thresholds to match other platforms (Ben)
    clean up mem freq calculation (Ben)
    clean up debug prints (Ben)
v4: move defines from punit patch (Ville)
v5: remove writes to nonexistent regs (Jesse)
    put RP and RC regs together (Jesse)
    fix RC6 enable (Jesse)
v6: use correct fuse reads from NC (Jesse)
    split out min/max funcs for use in sysfs (Jesse)
    add debugfs & sysfs freq controls (Jesse)
v7: update with Ben's hw_max changes (Jesse)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net> (v6)
[danvet: Follow checkpatch sugggestion to use min_t to avoid casting
fun.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>