drm/i915: Decouple cdclk calculation from modeset checks

drm/i915: Decouple cdclk calculation from modeset checks

We need to calculate cdclk after watermarks/ddb has been calculated
as with recent hw CDCLK needs to be adjusted accordingly to DBuf
requirements, which is not possible with current code organization.

Setting CDCLK according to DBuf BW requirements and not just rejecting
if it doesn't satisfy BW requirements, will allow us to save power when
it is possible and gain additional bandwidth when it's needed - i.e
boosting both our power management and perfomance capabilities.

This patch is preparation for that, first we now extract modeset
calculation from modeset checks, in order to call it after wm/ddb
has been calculated.

v2: - Extract only intel_modeset_calc_cdclk from intel_modeset_checks
      (Ville Syrjälä)

v3: - Clear plls after intel_modeset_calc_cdclk

v4: - Added r-b from previous revision to commit message

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200519131117.17190-2-stanislav.lisovskiy@intel.com