i965/fs/skl+: Use ld2dms_w instead of ld2dms

i965/fs/skl+: Use ld2dms_w instead of ld2dms

In order to support 16x MSAA, skl+ has a wider version of ld2dms that
takes two parameters for the MCS data. The MCS data retrieved from the
ld_mcs instruction already returns 4 or 8 registers and is documented
to return zeroes for the mcsh value when the sample count is less than
16.

v2: Use get_lowered_simd_width to fall back to SIMD8 instructions when
    the message length would be too long in SIMD16.
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>