drm/i915/lspcon: convert to drm device based logging

Prefer drm device based logging. Do some related dev_priv->i915 and
dp->intel_dp renames while at it.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8f83a4de60be1a4a964aa4334204db95d2db3689.1642769982.git.jani.nikula@intel.com