drm/i915/display: some struct drm_i915_private *i915 conversions

Prefer struct drm_i915_private *i915 over struct drm_device or dev_priv.

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/1bb84c4fffde5025ddc411148d529381a587e1e1.1655372759.git.jani.nikula@intel.com