gpio: omap: get rid of the conditional PM runtime calls

OMAP GPIO driver is checking !BANK_USED() used condition before calling PM
runtime API, because of PM runtime calls in
omap2_gpio_prepare/resume_for_idle(). It's not required any more since
"omap gpio add level idle, cpu_pm and drop runtime_irq_safe" series [1]
from Tony Lindgren was accepted and PM runtime management was enabled in
IRQ chip core by commit be45beb2 ("genirq: Add runtime power management
support for IRQ chips") .

As result safely drop !BANK_USED() checks from omap_gpio_request/free(),
omap_gpio_irq_bus_lock/unlock() and enable PM runtime management for OMAP
GPIO IRQ chip.

[1] https://www.spinics.net/lists/arm-kernel/msg677583.html


Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>