msm: mdss: dsi: use right mask to confirm dma transfer done

When dma tx timeout happens, the internal check to verify state of
hardware is actually checking for DMA interrupt enabled bit instead of
the actual state of DMA done.

Change-Id: I2c03e3e405289c223358b82e37e49203d0d0978b
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>