drm/i915: Set softmin frequency on idle->busy transition

If the GPU load is low enough, it's possible that we'll be stuck at idle
frequency rather than transition into softmin frequency requested by
userspace.

v2: Use intel_set_rps, drop vlv_set_idle
v3: Back to vlv_set_idle, clamp to valid range
v4: Place intel_set_rps at the end

References: https://bugs.freedesktop.org/show_bug.cgi?id=89728


Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1466416707-12075-1-git-send-email-michal.winiarski@intel.com