UPSTREAM: 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


(cherry picked from commit 0fa64765)
Signed-off-by: Lee Shawn C <shawn.c.lee@intel.com>

BUG=b:366539867
TEST=DP-ALT displays should work fine

Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/5875367


Reviewed-by: Sean Paul <sean@poorly.run>
Tested-by: Shawn C Lee <shawn.c.lee@intel.com>
Commit-Queue: Shawn C Lee <shawn.c.lee@intel.com>
Reviewed-by: Drew Davenport <ddavenport@chromium.org>
Change-Id: I82acbeeb85a365fd6890285214abc2dd295c47cc
Signed-off-by: Hubert Mazur <hmazur@google.com>
Signed-off-by: Tomasz Nowicki <tnowicki@google.com>