drm/vc4: select PM

commit  39cf7db9b384a651d481d0b04d9dee540624302a from
https://github.com/raspberrypi/linux.git rpi-5.15.y

We already depend on runtime PM to get the power domains and clocks for
most of the devices supported by the vc4 driver, so let's just select it
to make sure it's there, and remove the ifdef.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Meng Li <Meng.Li@windriver.com>