drm/msm/dsi: Simplify the code to get the number of read byte

During cmd rx, only new versions of H/W provide register to read back
the real number of byte returned by panel. For the old versions, reading
this register will not get the right number. In fact, we only need to
assume the returned data is the same size as we expected, because later
we will check the data type to detect error.

Signed-off-by: Hai Li <hali@codeaurora.org>