i965/vec4: Add a new dst_reg constructor accepting a brw_reg_type

i965/vec4: Add a new dst_reg constructor accepting a brw_reg_type

This is useful for the upcoming texture support in NIR->vec4 pass,
as we found several cases where the brw_type is available, but not
the glsl_type.

Without this new constructor, the alternative would be:
dst_reg reg(MRF, <reg>)
reg.type = <brw_type>
reg.writemask = <mask>

Adding a new constructor makes code easier to read.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>