[intel-i915] Add units to frequency values; Use kHz for pixel rates internally.

This change:
- Adds units to frequency values in method names
  (LoadPixelRateForTranscoderKhz) and arguments (ComputeDdiPllConfig,
  CheckPixelRate)

- Changes all "10khz" used internally used for pixel clock calculation
  to "khz". All the arithmetics are modified accordingly.

Test: display-tool squares 1 on NUC11 with HDMI display / DP display.
Test: intel-i915-test

Bug: 130662
Change-Id: Ib81bb0b8eaf65669291672d4136179d5ff1e5256
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/926232
Fuchsia-Auto-Submit: Yilong Li <liyl@google.com>
Reviewed-by: Victor Costan <costan@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>