i915: fix broken indirect constant buffer reads

i915: fix broken indirect constant buffer reads

The READ message's msg_control value can be 0 or 1 to indicate that the
Oword should be read into the lower or upper half of the target register.
It seems that the other half of the register gets clobbered though.  So
we read into two dest registers then use a MOV to combine the upper/lower
halves.