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>