drm/msm: dsi: Constify dsi_host_ops

The only usage of dsi_host_ops is to assign its address to the ops field
in the mipi_dsi_host struct, which is a const pointer. Make it const to
allow the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>