drm/i915/fbc: Extract intel_fbc_max_cfb_height()

Pull the code to determine the maximum CFB height
into a separate function.

To make this work we need to declare an explicit max height
for all older platforms as well. But that is actually just
the max plane height as pre-HSW hardware supposedly doesn't
have the trick of leaving the extra lines uncompressed.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240705145254.3355-12-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>