cxl/pci: Skip irq features if MSI/MSI-X are not supported

CXL 3.1 Section 3.1.1 states:

	"A Function on a CXL device must not generate INTx messages if
	that Function participates in CXL.cache protocol or CXL.mem
	protocols."

The generic CXL memory driver only supports devices which use the
CXL.mem protocol.  The current driver attempts to allocate MSI/MSI-X
vectors in anticipation of their need for mailbox interrupts or event
processing.  However, the above requirement does not require a device to
support interrupts, only that they use MSI/MSI-X.  For example, a device
may disable mailbox interrupts and either be configured for firmware
first or skip event processing and function.

Dave Larsen reported that the following Intel / Agilex card does not
support interrupts on function 0.

	CXL: Intel Corporation Device 0ddb (rev 01) (prog-if 10 [CXL Memory Device (CXL 2.x)])

Rather than fail device probe if interrupts are not supported; flag that
irqs are not enabled and avoid features which require interrupts.
Emit messages appropriate for the situation to aid in debugging should
device behavior be unexpected due to a failure to allocate vectors.

Note that it is possible for a device to have host based event
processing through polling.  However, the driver does not support
polling and it is not anticipated to be generally required.  Leave that
functionality to a future patch if such a device comes along.

Reported-by: Dave Larsen <[email protected]>
Reviewed-by: Dave Jiang <[email protected]>
Reviewed-by: Fan Ni <[email protected]>
Signed-off-by: Ira Weiny <[email protected]>
Reviewed-and-tested-by: Davidlohr Bueso <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Dan Williams <[email protected]>

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