drm/i915/pmu: Support multiple GPUs

With discrete graphics system can have both integrated and discrete GPU
handled by i915.

Currently we use a fixed name ("i915") when registering as the uncore PMU
provider which stops working in this case.

To fix this we add the PCI device name string to non-integrated devices
handled by us. Integrated devices keep the legacy name preserving
backward compatibility.

v2:
 * Detect IGP and keep legacy name. (Michal)
 * Use PCI device name as suffix. (Michal, Chris)

v3:
 * Constify the name. (Chris)
 * Use pci_domain_nr. (Chris)

v4:
 * Fix kfree_const usage. (Chris)

v5:
 * kfree_const does not work for modules. (Chris)
 * Changed is_igp helper to take i915.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191016093802.12483-1-tvrtko.ursulin@linux.intel.com