drm/gma500: Pass GPIO for Intel MID using descriptors

drm/gma500: Pass GPIO for Intel MID using descriptors

The GMA500 driver is using the legacy GPIO API to fetch
three optional display control GPIO lines from the SFI
description used by the Medfield platform.

Switch this over to use GPIO descriptors and delete the
custom platform data.

We create three new static locals in the tc35876x bridge
code but it is hardly any worse than the I2C client static
local already there: I tried first to move it to the DRM
driver state container but there are workarounds for
probe order in the code so I just stayed off it, as the
result is unpredictable.

People wanting to do a more throrugh and proper cleanup
of the GMA500 driver can work on top of this, I can't
solve much more since I don't have access to the hardware,
I can only attempt to tidy up my GPIO corner.

Cc: Daniel Stone <daniels@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191206094301.76368-1-linus.walleij@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>