drm/i915/perf: Simply use stream->ctx

Earlier code used exclusive_stream to check for user passed context.
Simplify this by accessing stream->ctx.

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221026222102.5526-7-umesh.nerlige.ramappa@intel.com