i965/blorp_clear: Use memcpy instead of assignment to copy clear value

i965/blorp_clear: Use memcpy instead of assignment to copy clear value

Similar to the problem described in 2c50212b14da27de4e3, if we copy the clear
value through a regular assignment via a floating point value, then if an
integer clear value is being used that happens to contain a signalling NaN
value then it would get converted to a quiet NaN when stored via the x87
floating-point registers. This would corrupt the integer value. Instead we
should use a memcpy to ensure the exact bit representation is preserved.

This bug can be triggered on 32-bit builds with optimisations by using an
integer clear color with a value like 0x7f817f81.

Reviewed-by: Matt Turner <mattst88@gmail.com>