drm/msm/dp: Simplify dp_debug_init() and dp_debug_get()

drm/msm/dp: Simplify dp_debug_init() and dp_debug_get()

dp_debug_init() always returns 0. So, make it a void function and simplify
the only caller accordingly.

While at it remove a useless 'rc' initialization in dp_debug_get()

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/dc2d6f535379dd38a5e3f9ba502f1f2b3d1f56b7.1640201523.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>