drm/i915: Read CHV_PLL_DW8 from the correct offset

drm/i915: Read CHV_PLL_DW8 from the correct offset

We accidentally pass 'pipe' instead of 'port' to CHV_PLL_DW8() and
with PIPE_C we end up at register offset 0x8320 which isn't the
0x8020 we wanted. Fix it.

The problem was fortunately caught by the sanity check in vlv_dpio_read():
WARNING: CPU: 1 PID: 238 at ../drivers/gpu/drm/i915/intel_sideband.c:200 vlv_dpio_read+0x77/0x80 [i915]()
DPIO read pipe C reg 0x8320 == 0xffffffff

The problem got introduced with this commit:
 commit 71af07f91f12bbab96335e202c82525d31680960
 Author: Vijay Purushothaman <vijay.a.purushothaman@linux.intel.com>
 Date:   Thu Mar 5 19:33:08 2015 +0530

    drm/i915: Update prop, int co-eff and gain threshold for CHV

Cc: Vijay Purushothaman <vijay.a.purushothaman@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>