drm/sun4i: dsi: delete unnecessary IS_ERR() checks

The "dsi->bus_clk" pointer cannot be an error pointer at this point.
The check is confusing and unnecessary.  Delete it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/YmqH71MtoGn2AXUg@kili