drm/i915: Make intel_bw_check_sagv_mask() internal to intel_bw.c

drm/i915: Make intel_bw_check_sagv_mask() internal to intel_bw.c

The only thing between the current intel_bw_check_sagv_mask() call
site and intel_bw_atomic_check() is skl_wm_add_affected_planes()
which no longer depends on the sagv mask, so we can make life
a lot less confusing by calling intel_bw_check_sagv_mask() from
intel_bw_atomic_check() instead.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250326162544.3642-12-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>