drm/msm/dp: Drop unused dp_debug struct

drm/msm/dp: Drop unused dp_debug struct

The members of struct dp_debug are no longer used, so the only purpose
of this struct is as a type of the return value of dp_debug_get(), to
signal success/error.

Drop the struct in favor of signalling the result of initialization
using an int, then merge dp_debug_get() with dp_debug_init() to avoid
the unnecessar boilerplate code.

Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/585343/
Link: https://lore.kernel.org/r/20240328-msm-dp-cleanup-v2-1-a5aed9798d32@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>