drm/amd/display: Fix unused variable warning

Some of the newly added code is hidden inside of #ifdef
blocks, but one variable is unused when debugfs is disabled:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:8370:8: error: unused variable 'configure_crc' [-Werror,-Wunused-variable]

Change the #ifdef to an if(IS_ENABLED()) check to fix the warning
and avoid adding more #ifdefs.

Fixes: c920888c

 ("drm/amd/display: Expose new CRC window property")
Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>