drm/i915/fbdev: add intel_fbdev_framebuffer() helper

Wrap accessing struct intel_fbdev guts in a helper.

v2: s/intel_fbdev_to_framebuffer/intel_fbdev_framebuffer/g (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220215122957.2755529-1-jani.nikula@intel.com