drm/vc4: hvs: Use pointer to HVS in HVS_READ and HVS_WRITE macros

Those macros are really about the HVS itself, and thus its associated
structure vc4_hvs, rather than the entire (virtual) vc4 device.

Let's change those macros to use the hvs pointer directly, and change
the calling sites accordingly.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20220331143744.777652-8-maxime@cerno.tech