msm: mdss: set default value of panel mode gpio to -EINVAL

Default value of panel mode gpio is set to 0 which is still a
valid gpio and preventing access to legitimate owners when not
provided explicitly in device tree file. This patch addresses
the issue by changing default value to -EINVAL.

Change-Id: Ifc34513d21323af9843f6bbc85a3afca79891357
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>