drm/i915/backlight: use VLV_DISPLAY_BASE for VLV/CHV backlight registers

Since the VLV/CHV backlight registers are only used on VLV/CHV, there's
no need to dynamically look up DISPLAY_MMIO_BASE(). We know it's
VLV_DISPLAY_BASE. Use it statically, 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/eb252083a56ec64b4fdb58d4d30abcf305a3a9c2.1670405587.git.jani.nikula@intel.com