drm/i915/gvt: Simplify case switch in intel_vgpu_ioctl

drm/i915/gvt: Simplify case switch in intel_vgpu_ioctl

We do not need a case switch to check cap_type_id in intel_vgpu_ioctl
for various reasons (it's impossible to hit the default case in the
current code, there's only one valid case to check, the error handling
code overlaps in both cases, etc.).  Simplify the case switch into a
single if statement.  This has the additional effect of simplifying the
error handling code.

Note that it is still currently impossible for
'if (cap_type_id == VFIO_REGION_INFO_CAP_SPARSE_MMAP)'
to fail, but we should still guard against the possibility of this
changing in the future.

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw.linux@gmail.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/20250918214515.66926-2-jonathan.cavitt@intel.com