drm/i915/backlight: drop DISPLAY_MMIO_BASE() use from backlight registers

None of the remaining backlight registers that use DISPLAY_MMIO_BASE()
are used on VLV/CHV, which are the only platforms that have non-zero
base. Just drop the DISPLAY_MMIO_BASE() use, reducing the implicit
dev_priv references.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/75ae3f2945912f908df2444d4f0ab97a23b89897.1670405587.git.jani.nikula@intel.com