drm/vc4: Convert register accessors to FIELD_*

The VC4_SET_FIELD and VC4_GET_FIELD are reimplementing most of the logic
already defined in FIELD_SET and FIELD_GET. Let's convert the vc4 macros to
use the FIELD_* macros.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200703135713.985810-1-maxime@cerno.tech