drm/amd/display: Add conditional around function

When CONFIG_DRM_AMD_DC_DCN is not set, the function
'dm_helpers_enable_periodic_detection' doesn't have its prototype defined,
causing the following warning:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_helpers.c:805:6:
warning: no previous prototype for function 'dm_helpers_enable_periodic_detection' [-Wmissing-prototypes]
   void dm_helpers_enable_periodic_detection(struct dc_context *ctx, bool enable)
        ^
   drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_helpers.c:805:1:
note: declare 'static' if the function is not intended to be used outside
of this translation unit
   void dm_helpers_enable_periodic_detection(struct dc_context *ctx, bool enable)
   ^
   static
   1 warning generated.

This commit silences this warning by adding a conditional directive
around the mentioned function, and also corrects a small spelling error.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>