drm/i915/guc: Add non blocking CTB send function

Add non blocking CTB send function, intel_guc_send_nb. GuC submission
will send CTBs in the critical path and does not need to wait for these
CTBs to complete before moving on, hence the need for this new function.

The non-blocking CTB now must have a flow control mechanism to ensure
the buffer isn't overrun. A lazy spin wait is used as we believe the
flow control condition should be rare with a properly sized buffer.

The function, intel_guc_send_nb, is exported in this patch but unused.
Several patches later in the series make use of this function.

v2:
 (Michal)
  - Use define for H2G room calculations
  - Move INTEL_GUC_SEND_NB define
 (Daniel Vetter)
  - Use msleep_interruptible rather than cond_resched
v3:
 (Michal)
  - Move includes to following patch
  - s/INTEL_GUC_SEND_NB/INTEL_GUC_CT_SEND_NB/g
v4:
 (John H)
  - Update comment, add type local variable

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708162055.129996-5-matthew.brost@intel.com