drm/i915/fbdev: hide struct intel_fbdev in intel_fbdev.c

As all access to struct intel_fbdev guts is nicely stowed away in
intel_fbdev.c, we can hide the struct definition there too.

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/20220215122957.2755529-2-jani.nikula@intel.com