drm/i915/dp: fix spelling mistakes: "seqeuncer" and "seqeuencer"

Trivial fix to spelling mistakes in WARN warning message text and
in comments:

"seqeuncer", "seqeuencer" -> "sequencer"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180509101606.17483-1-colin.king@canonical.com