omapfb: panel-sharp-ls037v7dw01: fix check of gpio_to_desc() return value

The change fixes a check of gpio_to_desc() return value, the function
returns either a valid pointer to struct gpio_desc or NULL, this makes
IS_ERR() check invalid and may lead to a NULL pointer dereference in
runtime.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>