dim+qf: fix shellcheck SC2209

Seems like a fairly reasonable thing to check to me, so lets appease
it.

This new failure is:

SC2209: Use var=$(command) to assign output (or quote to assign string).

v2: Improve commit message (Rodrigo)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>