drm/i915/cdclk: Extract bxt_cdclk_ctl()

Extract logic for deriving the value for CDCLK_CTL into bxt_cdclk_ctl().
This makes the code better readable and will be used later in
bxt_sanitize_cdclk().

v2:
  - Improve body of commit message to be more self-contained.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240105140538.183553-3-gustavo.sousa@intel.com