UPSTREAM: 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


(cherry picked from commit 2e13b5bb)
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/+/5875366


Reviewed-by: Drew Davenport <ddavenport@chromium.org>
Tested-by: Shawn C Lee <shawn.c.lee@intel.com>
Commit-Queue: Shawn C Lee <shawn.c.lee@intel.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Change-Id: I4dd6c1e79d078d6740cec118c0401fbe8ae0c9ec
Signed-off-by: Hubert Mazur <hmazur@google.com>
Signed-off-by: Tomasz Nowicki <tnowicki@google.com>