drm/radeon: Fix integer overflow in radeon_cs_parser_init

[ Upstream commit f828b681 ]

The type of size is unsigned, if size is 0x40000000, there will be an
integer overflow, size will be zero after size *= sizeof(uint32_t),
will cause uninitialized memory to be referenced later

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: hackyzh002 <hackyzh002@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>