drm/i915: Fix g4x fbc watermark enable

'level' here means the highest level we can't use, so when checking
the fbc watermarks we need a -1 to get at the last enabled level.

While at if refactor the code a bit to declutter
g4x_compute_pipe_wm().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429101034.8208-12-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>