arm64: simplify dma_get_ops

Including linux/acpi.h from asm/dma-mapping.h causes tons of compile-time
warnings, e.g.

 drivers/isdn/mISDN/dsp_ecdis.h:43:0: warning: "FALSE" redefined
 drivers/isdn/mISDN/dsp_ecdis.h:44:0: warning: "TRUE" redefined
 drivers/net/fddi/skfp/h/targetos.h:62:0: warning: "TRUE" redefined
 drivers/net/fddi/skfp/h/targetos.h:63:0: warning: "FALSE" redefined

However, it looks like the dependency should not even there as
I do not see why __generic_dma_ops() cares about whether we have
an ACPI based system or not.

The current behavior is to fall back to the global dma_ops when
a device has not set its own dma_ops, but only for DT based systems.
This seems dangerous, as a random device might have different
requirements regarding IOMMU or coherency, so we should really
never have that fallback and just forbid DMA when we have not
initialized DMA for a device.

This removes the global dma_ops variable and the special-casing
for ACPI, and just returns the dma ops that got set for the
device, or the dummy_dma_ops if none were present.

The original code has apparently been copied from arm32 where we
rely on it for ISA devices things like the floppy controller, but
we should have no such devices on ARM64.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[catalin.marinas@arm.com: removed acpi_disabled check in arch_setup_dma_ops()]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>