drm/msm/dsi: Report PHY errors only when they really occur

DSI PHY errors are falsely reported whenever a dsi error occurs. This is
because DSI_DLN0_PHY_ERR isn't only used as a status register, but also
used to mask PHY errors. Currently, we end up reading the mask bits too
and therefore always report errors.

Ignore the register mask bits and check for only the status/clear bits.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>