drm/i915/bxt: Explicitly clear the Turbo control register

drm/i915/bxt: Explicitly clear the Turbo control register

As a part of WaGsvDisableTurbo, Driver makes an early exit from the
Gen9 Turbo enabling function, so doesn't program the Turbo Control register.
But BIOS could leave the Hw Turbo as enabled, so need to explicitly clear
out the Control register just to avoid inconsitency with debugfs
interface, which will show Turbo as enabled only and that is not expected
after adding the WaGsvDisableTurbo. Apart from this there is no problem
even if the Turbo is left enabled in the Control register, as the Up/Down
interrupts would remain masked.

v2: Add explicit clearing of Turbo Control register to *_disable_rps()
    also for the similar consistency (Chris)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1461350146-23454-2-git-send-email-akash.goel@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>