drm/radeon: integer overflow in radeon_mode_dumb_create()

Similar to the handling of amdgpu_mode_dumb_create in commit 54ef0b54
("drm/amdgpu: integer overflow in amdgpu_mode_dumb_create()"),
we thought a patch might be needed here as well.

args->size is a u64.  arg->pitch and args->height are u32.  The
multiplication will overflow instead of using the high 32 bits as
intended.

Signed-off-by: Xiaohui Zhang <xiaohuizhang@ruc.edu.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>