drm: msm: Fix return type of dsi_mgr_connector_mode_valid for kCFI

I was hitting kCFI crashes when building with clang, and after
some digging finally narrowed it down to the
dsi_mgr_connector_mode_valid() function being implemented as
returning an int, instead of an enum drm_mode_status.

This patch fixes it, and appeases the opaque word of the kCFI
gods (seriously, clang inlining everything makes the kCFI
backtraces only really rough estimates of where things went
wrong).

Thanks as always to Sami for his help narrowing this down.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Alistair Delva <adelva@google.com>
Cc: Amit Pundir <amit.pundir@linaro.org>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: freedreno@lists.freedesktop.org
Cc: clang-built-linux@googlegroups.com
Signed-off-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>