mesa/main: Fix overflow in validation of DispatchComputeGroupSizeARB

An uint64_t can store the result of multiplying two GLuint (uint32_t), so use that property to check for overflow when calculating the total.

Change the error message so we don't need to care about the actual total -- which means we don't need a larger than 64-bit value to hold it.

Fixes: 45ab63c0 ("mesa/main: add support for ARB_compute_variable_groups_size")