dim: Don't escape forward slash for grep

GNU grep 3.8 started to complain about backslashes where
they're not needed:
 grep: warning: stray \ before /
Get rid of them.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>