msm: mdss: Fix destination bpp on writeback panel

Default bpp for destination format is set if mixer type
for WB is MIXER_TYPE_WRITEBACK. Ensure that correct bpp
is fetched from format type.

Change-Id: Iff7cf58363a83e9a4999115382c5551110104ead
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>