drm/i915/guc: Drop guc->interrupts.enabled

Drop the variable guc->interrupts.enabled as this variable is just
leading to bugs creeping into the code.

e.g. A full GPU reset disables the GuC interrupts but forgot to clear
guc->interrupts.enabled, guc->interrupts.enabled being true suppresses
interrupts from getting re-enabled and now we are broken.

It is harmless to enable interrupt while already enabled so let's just
delete this variable to avoid bugs like this going forward.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603051630.2635-7-matthew.brost@intel.com