drm/vc4: Optimise vc4_hvs_dlist_free_work to only read frcnt and active once

vc4_hvs_dlist_free_work was iterating through the list of stale
dlist entries and reading the frame count and active flags from
the hardware for each one.

Read the frame count and active flags once, and then use the
cached value in the loop.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>