drm/i915/fbc: Handle 16bpp compression limit better

The limit++ for the 16bpp case is nonsense since the
compression limit is always supposed to be power of two.
Replace it with <<=1.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210610183237.3920-6-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>