drm/radeon/r600_cs: Fix possible int overflow in r600_packet3_check()

It is possible, although unlikely, that an integer overflow will occur
when the result of radeon_get_ib_value() is shifted to the 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.

Signed-off-by: Igor Artemiev <Igor.A.Artemiev@mcst.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>