drm/radeon/r600_cs: Fix possible int overflows in r600_cs_check_reg()

While improbable, there may be a chance of hitting integer
overflow when the result of radeon_get_ib_value() gets shifted
left.

Avoid it by casting one of the operands to larger data type (u64).

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 1729dd33 ("drm/radeon/kms: r600 CS parser fixes")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>