i965: Make brw_imm_vf4() take 8-bit restricted floats.

i965: Make brw_imm_vf4() take 8-bit restricted floats.

This partially reverts commit bbf8239f92ecd79431dfa41402e1c85318e7267f.

I didn't like that commit to begin with -- computing things at compile
time is fine -- but for purposes of verifying that the resulting values
are correct, looking up 0x00 and 0x30 in a table is a lot better than
evaluating a recursive function.

Anyway, by making brw_imm_vf4() take the actual 8-bit restricted floats
directly (instead of only integral values that would be converted to
restricted float), we can use this function as a replacement for the
vector float src_reg/fs_reg constructors.

brw_float_to_vf() is not currently an inline function, so it will not be
evaluated at compile time. I'll address that in a follow-up patch.

Reviewed-by: Emil Velikov <emil.velikov@collabora.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>