drm/msm/dpu: use drm_rect_fp_to_int()

Use the drm_rect_fp_to_int() helper instead of using the hand-written
code.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/621476/
Link: https://lore.kernel.org/r/20241025-dpu-virtual-wide-v6-1-0310fd519765@linaro.org