drm/xe/irq: Rename fuse mask variables

drm/xe/irq: Rename fuse mask variables

It's confusing to refer to some masks as the interrupt masks and others
as the fuse masks. Rename the fuse one to make it clearer.

Note that the most important role they play here is that the call
to xe_hw_engine_mask_per_class() will not only limit the engines
according to the fuses, but also by what is available in the specific
architecture - the latter is more important information to know what
interrupts should be enabled. Add a comment about that.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20251016-xe3p-v3-17-3dd173a3097a@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>