drm/vc4/vc4_hdmi_regs: Mark some data sets as __maybe_unused

The alternative is to move them into the source file that uses then,
but they are large and intrusive, so that strategy is being avoided.

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/vc4/vc4_hdmi_regs.h:282:39: warning: ‘vc5_hdmi_hdmi1_fields’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/vc4/vc4_hdmi_regs.h:206:39: warning: ‘vc5_hdmi_hdmi0_fields’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/vc4/vc4_hdmi_regs.h:145:39: warning: ‘vc4_hdmi_fields’ defined but not used [-Wunused-const-variable=]

Cc: Eric Anholt <eric@anholt.net>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-38-lee.jones@linaro.org