amdgpu/pm: Clarify documentation of error handling in send_smc_mesg

Clarify the smu_cmn_send_smc_msg_with_param documentation to mention two
cases exist where messages are silently dropped with no error returned.
These cases occur in unusual situations where either:
 1. the message type is not allowed to a virtual GPU, or
 2. a PCI recovery is underway and the HW is not yet in sync with the SW

For more details see
 commit 4ea5081c ("drm/amd/powerplay: enable SMC message filter")
 commit bf36b52e ("drm/amdgpu: Avoid accessing HW when suspending SW state")

(v2)
  Reworked with suggestions from Luben & Paul

(v3)
  Updated wording as per Luben's feedback
  Corrected error stating all messages denied on virtual GPU
  (each GPU has mask of which messages are allowed)
Signed-off-by: Darren Powell <darren.powell@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/pm/swsmu/smu_cmn.c

@@ -356,9 +356,11 @@ int smu_cmn_wait_for_response(struct smu_context *smu)
  * completion of the command, and return back a value from the SMU in
  * @read_arg pointer.
  *
- * Return 0 on success, -errno on error, if we weren't able to send
- * the message or if the message completed with some kind of
- * error. See __smu_cmn_reg2errno() for details of the -errno.
+ * Return 0 on success, -errno when a problem is encountered sending
+ * message or receiving reply. If there is a PCI bus recovery or
+ * the destination is a virtual GPU which does not allow this message
+ * type, the message is simply dropped and success is also returned.
+ * See __smu_cmn_reg2errno() for details of the -errno.
  *
  * If we weren't able to send the message to the SMU, we also print
  * the error to the standard log.