drm/i915: Perf_limit_reasons are only available for Gen11+

Register GT0_PERF_LIMIT_REASONS (0x1381a8) is available only for
Gen11+. Therefore ensure perf_limit_reasons sysfs/debugfs files are created
only for Gen11+. Otherwise on Gen < 5 accessing these files results in the
following oops:

<1> [88.829420] BUG: unable to handle page fault for address: ffffc90000bb81a8
<1> [88.829438] #PF: supervisor read access in kernel mode
<1> [88.829447] #PF: error_code(0x0000) - not-present page

Bspec: 20008
Bug: https://gitlab.freedesktop.org/drm/intel/-/issues/6863


Fixes: fe597966 ("drm/i915/debugfs: Add perf_limit_reasons in debugfs")
Fixes: fa68bff7 ("drm/i915/gt: Add sysfs throttle frequency interfaces")
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220919162401.2077713-1-ashutosh.dixit@intel.com