drm/msm/dpu: Fix sm8250_mdp register length

The downstream dts lists this value as 0x494, and not
0x45c.

Fixes: af776a3e

 ("drm/msm/dpu: add SM8250 to hw catalog")
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Link: https://lore.kernel.org/r/20210628085033.9905-1-robert.foss@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>