i965/fs: Fix fs_inst::regs_read() for uniform pull constant loads

i965/fs: Fix fs_inst::regs_read() for uniform pull constant loads

Previously, fs_inst::regs_read() fell back to depending on the register
width for the second source.  This isn't really correct since it isn't a
SIMD8 value at all, but a SIMD4x2 value.  This commit changes it to
explicitly be always one register.

v2: Use mlen for determining the number of registers read

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Acked-by: Francisco Jerez <currojerez@riseup.net>