drm/msm: Fix wrong size calculation

drm/msm: Fix wrong size calculation

For example, memory-region in .dts as below,
	reg = <0x0 0x50000000 0x0 0x20000000>

We can get below values,
struct resource r;
r.start = 0x50000000;
r.end	= 0x6fffffff;

So the size should be:
size = r.end - r.start + 1 = 0x20000000

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Fixes: 34966f288423 ("drm/msm: add support for "stolen" mem")
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20220112123334.749776-1-xianting.tian@linux.alibaba.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>