drm: bridge/dw_hdmi: rename dw_hdmi_phy_enable_power()

dw_hdmi_phy_enable_power() is not about enabling and disabling power.
It is about allowing or preventing power-down mode being entered - the
register is documented as "Power-down enable (active low 0b)."

This can be seen as the bit has no effect when the HDMI phy is
operational on iMX6 hardware.

Rename the function to dw_hdmi_phy_enable_powerdown() to reflect the
documentation, make it take a bool for the 'enable' argument, and invert
the value to be written.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>