drm/msm/dp: use ARRAY_SIZE for calculating num_descs

If for some reason the msm_dp_config::descs array starts from non-zero
index or contains the hole, setting the msm_dp_config::num_descs might
be not that obvious and error-prone. Use ARRAY_SIZE to set this field
rather than encoding the value manually.

Reported-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/491399/
Link: https://lore.kernel.org/r/20220627165413.657142-1-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>