drm/i915: preserve the PBC bits of TRANS_CHICKEN2

Bits 30 and 24:0 are PBC, so don't zero them. Some of the other bits
are being zeroed, but I couldn't find a reason for this, so leave them
as they are for now to avoid regressions.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
[danvet: Delete the redudant #define that Imre spotted in his review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>