drm/i915/perf: Refactor oa object to better manage resources

The oa object manages the oa buffer and must be allocated when the user
intends to read performance counter snapshots. This can be achieved by
making the oa object part of the stream object which is allocated when a
stream is opened by the user.

Attributes in the oa object that are gen-specific are moved to the perf
object so that they can be initialized on driver load.

The split provides a better separation of the objects used in perf
implementation of i915 driver so that resources are allocated and
initialized only when needed.

v2: Fix checkpatch warnings
v3: Addressed Lionel's review comment
v4: Rebase
v5: Fix rebase/merge issue with ratelimit_state_init

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190806233002.984-1-umesh.nerlige.ramappa@intel.com