msm: mdss: handle DSI command rx for less than 8 bytes requested

Currently, for long read response commands with rlen less than
8, there are padding bytes being expected to be sent by the panel.
However, there might be some panels which do not support extra
padding bytes. Due to this, the read response might not be stored
in the first byte of the rx buffer.

The RDBK_DATA_CTRL register is available from DSI6G v1.1.0 and
above. Make use of this register to check the number of bytes
returned by the panel. Based on this, calculate the offset of
the first valid byte of the response in the rx buffer. For
targets which do not have this register, calculate the offset
based on the software state of the read length variable.

Change-Id: I94d3f6d7a453f925bdfd967cdb23cf6aab1ac3a8
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>