drm/i915/cnl: Fix RMW on ddi vswing sequence.

Paulo noticed that we were missing few bits clear
before writing values back to the register on
these RMW MMIO operations.

v2: Remove "POST_" from CURSOR_COEFF_MASK. (Paulo).
v3: Remove unnecessary braces. (Jani).

Fixes: cf54ca8b ("drm/i915/cnl: Implement voltage swing sequence.")
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1497897572-22520-1-git-send-email-rodrigo.vivi@intel.com