pan/bi: Implement f2f16{_rtz, _rtne}

Float conversions with explicit rounding modes are required for OpenCL,
as well as for Vulkan with the VK_KHR_16bit_storage extension (mandatory
in Vulkan 1.1). Since the hardware conversion instructions allow
configuring the round mode, this is easy to support :-)

Fixes test_half.vstore_half_rtz.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!17262>
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