drm/i915/display: Use helper to select C20 MPLLA/B

We used to select between MPLLA/B with the following

state->tx[0] & C20_PHY_USE_MPLLB

Since this is used a few places within C20 PLL setting,
let's introduce a helper function to clean up the code
a 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/20240105112243.224199-1-mika.kahola@intel.com