drm/{i915,xe}/panic: rename struct {i915,xe}_panic_data to struct intel_panic

Prepare for better shared interfaces between panic implementations. The
struct intel_panic remains an opaque data type, with unique
implementations in i915 and xe.

This allows us to change the panic data pointer from void * to struct
intel_panic *, helping type safety.

Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Maarten Lankhorst <dev@lankhorst.se>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/884ffc80c8b5fef1b92956e644a4e559560cc2ba.1756835342.git.jani.nikula@intel.com