r300: Correctly translate the value for the R300_CMD_WAIT command.

Previously, the R300_CMD_WAIT command would write the passed directly to the
hardware. However this is incorrect because the R300_WAIT_* values used are
internal interface values that do not map directly to the hardware.

The new function I have added translates the R300_WAIT_* values into appropriate
values for the hardware before writing the register.

Thanks to John Bridgman for pointing this out. :-)