drm/panel: sitronix-st7789v: Check display ID

A very basic debugging rule when a device is connected for the first
time is to access a read-only register which contains known data in
order to ensure the communication protocol is properly working. This
driver lacked any read helper which is often a critical piece for
speeding-up bring-ups.

Add a read helper and use it to verify the communication with the panel
is working as soon as possible in order to inform the user early if this
is not the case.

As this panel may work with no MISO line, the check is discarded in this
case. Upon error, we do not fail probing but just warn the user, in case
the DT description would be lacking the Rx bus width (which is likely on
old descriptions) in order to avoid breaking existing devices.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Tested-by: Sebastian Reichel <sre@kernel.org> # no MISO line
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230714013756.1546769-20-sre@kernel.org