drm/i915/display: Cleanup mplla/mpllb selection

The function intel_c20_use_mplla() is not really
widely used and can be replaced with the more suitable

pll->tx[0] & C20_PHY_USE_MPLLB

expression. Let's remove the intel_c20_use_mplla()
alltogether and replace mplla/mpllb selection by
checking mpllb bit.

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240102115741.118525-4-mika.kahola@intel.com