drm/i915/display: add a gelper to relative data rate handling

drm/i915/display: add a gelper to relative data rate handling

Add a helper to the relative data rate handling in skl_watermarks.c
where other similar functions are implemented. Also get rid of
use_min_ddb() and use use_minimal_wm0() instead to decide whether
the relative data rate can be returned as 0

v2: re-phrase the commit description (uma)

Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241121112726.510220-3-vinod.govindapillai@intel.com