drm/i915/gen6+: Clear upper data byte during PCODE write

The spec calls for the upper data byte to be cleared before most of the
PCODE write commands, for others like IPS control it doesn't say
anything about this byte. Let's clear it in case it's clobbered somehow,
especially that there are places where we only do a PCODE write without
a preceding PCODE read.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480346969-16121-1-git-send-email-imre.deak@intel.com