drm/vc4: crtc: Turn static const variable into a define

The hvs_latency_pix variable doesn't need to be a variable and can just be
defined.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/8535c679f79af8abaa1b7796261bfeda11f874fd.1590594512.git-series.maxime@cerno.tech