drm/i915: fix reassignment of variable "intel_dp->DP"

In intel_dp_mode_set we OR in the exact same bits twice at the same
spot. Kill one of the redundant assignments.

This little regression was introduced by:
commit 417e822d
Author: Keith Packard <keithp@keithp.com>
Date:   Tue Nov 1 19:54:11 2011 -0700

    drm/i915: Treat PCH eDP like DP in most places

	PCH eDP has many of the same needs as regular PCH DP connections,
	including the DP_CTl bit settings, the TRANS_DP_CTL register.

The reachable tag for this commit is: v3.1-5461-g417e822

Signed-off-by: Anhua Xu <anhua.xu@intel.com>
[danvet: Improved the commit message somewhat and ensured the diff is
clearer.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>