drivers: display: gd7965: fix build break

gd7965 broke after 42bbb30e:

/zephyr/drivers/display/gd7965.c: In function 'gd7965_init':
/zephyr/drivers/display/gd7965.c:379:23: error: passing argument 1 of
'device_is_ready' from incompatible pointer type
[-Werror=incompatible-pointer-types]
  379 |  if (!device_is_ready(&config->reset_gpio.port)) {
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~
      |                       |
      |                       const struct device * const*

Fix all the occurrences.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>