msm: mdss: dsi: fix dsi fifo underflow error status check

Remove obsolete dsi fifo error status check based on
CMD_MDP_FIFO_UNDERFLOW and check the correct status from
DLNx_HS_FIFO_UNDERFLOW bits in dsi fifo status register.

CRs-Fixed: 961817
Change-Id: I3664e51ffa0fc729e949d85c3d543ec338444a9d
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>