drm/i915/hdmi: prefer to_i915() over drm->dev_private to get at i915

drm->dev_private is to be avoided. Use to_i915() on the struct
drm_device pointer instead. Rename the affected local dev_priv variables
to i915 while at it.

Cc: Wambui Karuga <wambui.karugax@gmail.com>
Reviewed-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211134427.31605-1-jani.nikula@intel.com