drm/vc4: Update type of reg parameter in vc4_hdmi_{read,write}

drm/vc4: Update type of reg parameter in vc4_hdmi_{read,write}

Clang warns 100+ times in the vc4 driver along the lines of:

drivers/gpu/drm/vc4/vc4_hdmi_phy.c:518:13: warning: implicit conversion
from enumeration type 'enum vc4_hdmi_field' to different enumeration
type 'enum vc4_hdmi_regs' [-Wenum-conversion]
        HDMI_WRITE(HDMI_TX_PHY_POWERDOWN_CTL,
        ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~

The HDMI_READ and HDMI_WRITE macros pass in enumerators of type
vc4_hdmi_field but vc4_hdmi_write and vc4_hdmi_read expect a enumerator
of type vc4_hdmi_regs, causing a warning for every instance of this.
Update the parameter type so there is no more mismatch.

Fixes: 311e305fdb4e ("drm/vc4: hdmi: Implement a register layout abstraction")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1149
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200910170401.3857250-1-natechancellor@gmail.com