drm/xe/pxp: Clarify PXP queue creation behavior if PXP is not ready

drm/xe/pxp: Clarify PXP queue creation behavior if PXP is not ready

The expected flow of operations when using PXP is to query the PXP
status and wait for it to transition to "ready" before attempting to
create an exec_queue. This flow is followed by the Mesa driver, but
there is no guarantee that an incorrectly coded (or malicious) app
will not attempt to create the queue first without querying the status.
Therefore, we need to clarify what the expected behavior of the queue
creation ioctl is in this scenario.

Currently, the ioctl always fails with an -EBUSY code no matter the
error, but for consistency it is better to distinguish between "failed
to init" (-EIO) and "not ready" (-EBUSY), the same way the query ioctl
does. Note that, while this is a change in the return code of an ioctl,
the behavior of the ioctl in this particular corner case was not clearly
spec'd, so no one should have been relying on it (and we know that Mesa,
which is the only known userspace for this, didn't).

v2: Minor rework of the doc (Rodrigo)

Fixes: 72d479601d67 ("drm/xe/pxp/uapi: Add userspace and LRC support for PXP-using queues")
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250522225401.3953243-7-daniele.ceraolospurio@intel.com
(cherry picked from commit 21784ca96025b62d95b670b7639ad70ddafa69b8)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>