drm/amd/display: hide an unused variable

Without CONFIG_DEBUG_FS, we get a warning for an unused
variable:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:6020:33: error: unused variable 'source' [-Werror,-Wunused-variable]

Hide the variable in an #ifdef like its only users.

Fixes: 14b25846 ("drm/amd/display: add functionality to grab DPRX CRC entries.")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -6017,7 +6017,9 @@ static void amdgpu_dm_enable_crtc_interrupts(struct drm_device *dev,
 	struct drm_crtc *crtc;
 	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
 	int i;
+#ifdef CONFIG_DEBUG_FS
 	enum amdgpu_dm_pipe_crc_source source;
+#endif
 
 	for_each_oldnew_crtc_in_state(state, crtc, old_crtc_state,
 				      new_crtc_state, i) {