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

Prefer drm device based logging. Do some dev_priv->i915 conversions
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/ca6908452a63bd74a9c9d75ecd295182c80c7205.1642769982.git.jani.nikula@intel.com