drm/i915/dp: Helper for checking DDI_BUF_CTL Idle status

drm/i915/dp: Helper for checking DDI_BUF_CTL Idle status

Modify the helper to add a fixed delay or poll with timeout
based on platform specification to check for either Idle bit
set (DDI_BUF_CTL is idle for disable case)

v2:
* Use 2 separate functions or idle and active (Ville)
v3:
* Change the timeout to 16usecs (Ville)
v4:
* Change the timeout 8, follow spec (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200701221052.8946-1-manasi.d.navare@intel.com