ARM: cleanup: pwrdm_can_ever_lose_context() checking

pwrdm_can_ever_lose_context() is only ever called from the OMAP GPIO
code, and only with a pointer returned from omap_hwmod_get_pwrdm().
omap_hwmod_get_pwrdm() only ever returns NULL on error, so using
IS_ERR_OR_NULL() to validate the passed pointer is silly.  Use a
simpler !ptr check instead.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>