drm/i915/backlight: mass rename dev_priv to i915

With the implicit dev_priv usage gone, we can rename dev_priv to i915
throughout. Do some drive-by whitespace cleanups while at it.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Reviewed-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fc8b260bd8fa338edb312637f18ca7e6550d820d.1670405587.git.jani.nikula@intel.com