drm: bridge: dw-hdmi: Fix the name of the PHY reset macros

The PHY reset signal is controlled by bit PHYRSTZ in the MC_PHYRSTZ
register. The signal is active low on Gen1 PHYs and active high on Gen2
PHYs. The driver toggles the signal high then low, which is correct for
all currently supported platforms, but the register values macros are
incorrectly named. Replace them with a single macro named after the bit,
and add a comment to the source code to explain the behaviour.

The driver's behaviour isn't changed by this rename, the code will still
need to be fixed to support Gen1 PHYs.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170117082910.27023-19-laurent.pinchart+renesas@ideasonboard.com