simplefb: Disable and release clocks and regulators in destroy callback

simplefb gets unregister when a proper framebuffer driver comes in and
kicks it out. However the claimed clocks and regulators stay enabled
as they are only released in the platform device remove function, which
in theory would never get called.

Move the clock/regulator cleanup into the framebuffer destroy callback,
which gets called as part of the framebuffer unregister process.

Note this introduces asymmetry in how the resources are claimed and
released.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>