drm/i915: Unduplicate VLV signal level code

drm/i915: Unduplicate VLV signal level code

The logic for setting signal levels is used for both HDMI and DP with
small variations. But it is similar enough to put behind a function
called from the encoders.

v2: Remove unrelated MST changes due to rebase fumble. (Jim Bride)
    Fix typo in the commit message. (Jim Bride)

v3: Really fix the typo. (Jim)

Cc: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461761065-21195-8-git-send-email-ander.conselvan.de.oliveira@intel.com