mdss: display: making lpg channel and pwm period as optional

These properties need to be optional and used only when having an embedded
DP monitor, i.e. eDP.

When using the eDP driver with external DP monitor there is no need to
configure the fields for the external backlight since it is done by the
monitor itself.

Change-Id: I914a8c75acb406a900e9abd896253606c32e3601
Signed-off-by: Asaf Penso <apenso@codeaurora.org>