msm: mdss: correct bpp for TP10 in bandwidth calculations

Current driver uses a 1.5 bpp for bandwidth calculations
in any format with 420 chroma sample. This is wrong for
TP10 pixel format, where we need to use 2 bpp.
Fix this issue by making sure driver uses the correct
bpp for this pixel format.

Change-Id: I00c7349513fcb992565bf5ab1463900194a3d146
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>