drm: remove unnecessary return variable

Remove unnecessary variable `ret` in drm_dp_atomic_find_vcpi_slots()
only used to hold the function return value and have the function
return the value directly.
Issue found by coccinelle:
@@
local idexpression ret;
expression e;
@@

-ret =
+return
     e;
-return ret;

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191019071840.16877-1-wambui@karuga.xyz