drm/i915/dsi: Replace MIPI command error message with debug message

Error message indicating that the same MIPI command is sent
consecutively is perhaps too strongly said. Let's replace that as a
debug message instead.

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1504252261-28964-3-git-send-email-mika.kahola@intel.com