drm/i915: Extract {i9xx,i8xx,ilk,vlv,chv}_dpll()

drm/i915: Extract {i9xx,i8xx,ilk,vlv,chv}_dpll()

The *_compute_dpll() functions generally contain
two things:
- huge pile of inline code to calculate the DPLL
  register value
- a few calls to helpers to calculate the
  DPLL_MD and FP register values

Pull the DPLL register value calculations into a helpers
as well, so that *_compute_dpll() can focus on higher
level tasks.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240412182703.19916-11-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>