drm/i915: Update RAWCLK_FREQ register on VLV/CHV

I just noticed that VLV/CHV have a RAWCLK_FREQ register just like PCH
platforms. It lives in the display power well, so we should update it
when enabling the power well.

Interestingly the BIOS seems to leave it at the reset value (125) which
doesn't match the rawclk frequency on VLV/CHV (200 MHz). As always with
these register, the spec is extremely vague what the register does. All
it says is: "This is used to generate a divided down clock for
miscellaneous timers in display." Based on a quick test, at least AUX
and PWM appear to be unaffected by this.

But since the register is there, let's configure it in accordance with
the spec.

Note that we have to move intel_update_rawclk() to occur before we
touch the power wells, so that the dev_priv->rawclk_freq is already
populated when the disp2 enable hook gets called for the first time.
I think this should be safe to do on other platforms as well.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461768202-17544-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>