drm/i915/perf: Wean ourselves off dev_priv

Use the local uncore accessors for the GT rather than using the [not-so]
magic global dev_priv mmio routines. In the process, we also teach the
perf stream to use backpointers to the i915_perf rather than digging it
out of dev_priv.

v2: Rebase onto i915_perf_types.h

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20191007140812.10963-1-chris@chris-wilson.co.uk
Link: https://patchwork.freedesktop.org/patch/msgid/20191007210942.18145-1-chris@chris-wilson.co.uk