video: simple_panel: use regulator_set_enable_if_allowed

With the commit 4fcba5d5 ("regulator: implement basic reference
counter") the return value of regulator_set_enable may be EALREADY or
EBUSY for fixed/gpio regulators and may be further expanded on all
regulators.

Change to use the more relaxed regulator_set_enable_if_allowed to
continue if regulator already was enabled or disabled.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>