drm/i915/icl: Wait for header/payload credits release

Driver needs payload/header credits for sending any command
and data over DSI link. These credits are released once command
or data sent to link. This patch adds functions to wait for releasing
of payload and header credits.

As per BSPEC, driver needs to ensure that all of commands/data
has been dispatched to panel before the transcoder is enabled.
This patch implement those steps i.e. sending NOP DCS command,
wait for header/payload credit to be released etc.

v2 by Jani:
 - squash the credit wait helpers patch with the first user
 - pass dev_priv to the credit wait helpers
 - bikeshed credit helper names
 - wait for *at least* the current maximum number of credits
 - indentation fix
 - add helpers for credits available

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/84bc509beabf2a2d1324a9f2a67ab4ebe05b10a6.1540900289.git.jani.nikula@intel.com