drm/i915/guc/slpc: Provide sysfs for efficient freq

SLPC enables use of efficient freq at init by default. It is
possible for GuC to request frequencies that are higher than
the 'software' max if user has set it lower than the efficient
level.

Scenarios/tests that require strict fixing of freq below the efficient
level will need to disable it through this interface.

v2: Keep just one interface to toggle sysfs. With this, user will
be completely responsible for toggling efficient frequency if need
be. There will be no implicit disabling when user sets min < RP1 (Ashutosh)

v3: Remove unused label, review comments (Ashutosh)

v4: Toggle efficient freq usage in SLPC selftest and checkpatch fixes

v5: Review comments (Andi) and add a separate patch for selftest updates

Fixes: 95ccf312 ("drm/i915/guc/slpc: Allow SLPC to use efficient frequency")
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230426003942.1924347-1-vinay.belgaumkar@intel.com