drm/i915: Initialise min/max frequencies before updating RPS registers

The RPS register writing routines use the current value of min/max to
set certain limits and interrupt gating. If we set those afterwards, we
risk setting up the hw incorrectly and losing power management events,
and worse, trigger some internal assertions.

Reorder the calling sequences to be correct, and remove the then
unrequired clamping from inside set_rps(). And for a bonus, fix the bug
of calling gen6_set_rps() from Valleyview.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>