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

drivers/gpu/drm/vc4/vc4_hdmi_regs.h

@@ -142,7 +142,7 @@ struct vc4_hdmi_register {
 #define VC5_RAM_REG(reg, offset)	_VC4_REG(VC5_RAM, reg, offset)
 #define VC5_RM_REG(reg, offset)		_VC4_REG(VC5_RM, reg, offset)
 
-static const struct vc4_hdmi_register vc4_hdmi_fields[] = {
+static const struct vc4_hdmi_register __maybe_unused vc4_hdmi_fields[] = {
 	VC4_HD_REG(HDMI_M_CTL, 0x000c),
 	VC4_HD_REG(HDMI_MAI_CTL, 0x0014),
 	VC4_HD_REG(HDMI_MAI_THR, 0x0018),
@@ -203,7 +203,7 @@ static const struct vc4_hdmi_register vc4_hdmi_fields[] = {
 	VC4_HDMI_REG(HDMI_RAM_PACKET_START, 0x0400),
 };
 
-static const struct vc4_hdmi_register vc5_hdmi_hdmi0_fields[] = {
+static const struct vc4_hdmi_register __maybe_unused vc5_hdmi_hdmi0_fields[] = {
 	VC4_HD_REG(HDMI_DVP_CTL, 0x0000),
 	VC4_HD_REG(HDMI_MAI_CTL, 0x0010),
 	VC4_HD_REG(HDMI_MAI_THR, 0x0014),
@@ -279,7 +279,7 @@ static const struct vc4_hdmi_register vc5_hdmi_hdmi0_fields[] = {
 	VC5_CSC_REG(HDMI_CSC_34_33, 0x018),
 };
 
-static const struct vc4_hdmi_register vc5_hdmi_hdmi1_fields[] = {
+static const struct vc4_hdmi_register __maybe_unused vc5_hdmi_hdmi1_fields[] = {
 	VC4_HD_REG(HDMI_DVP_CTL, 0x0000),
 	VC4_HD_REG(HDMI_MAI_CTL, 0x0030),
 	VC4_HD_REG(HDMI_MAI_THR, 0x0034),