msm: mdss: hdmi: set the output format to RGB888

Set the HDMI panel's output format to RGB888 when the panel is
registered during probe time. This is the default configuration
for HDMI panels until there is a request from the framework to
change the output after connecting to a sink that supports an
alternative format e.g. YUV420.

CRs-Fixed: 1008555
Change-Id: Ic7c68c0025231208215c02e406044857eb2ebe04
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>