drm/panel: sitronix-st7789v: Clarify a definition

The Sitronix datasheet explains BIT(1) of the RGBCTRL register as the
DOTCLK/PCLK edge used to sample the data lines:

    “0” The data is input on the positive edge of DOTCLK
    “1” The data is input on the negative edge of DOTCLK

IOW, this bit implies a falling edge and not a high state. Correct the
definition to ease the comparison with the datasheet.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Tested-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230714013756.1546769-18-sre@kernel.org