i965: Add viewport extents (gen8) to batch decode

i965: Add viewport extents (gen8) to batch decode

0x00007da0:      0xc1da740e: SF_CLIP VP: guardband xmin = -27.306667
0x00007da4:      0x41da740e: SF_CLIP VP: guardband xmax = 27.306667
0x00007da4:      0x41da740e: SF_CLIP VP: guardband ymin = -23.405714
0x00007da8:      0xc1bb3ee7: SF_CLIP VP: guardband ymax = 23.405714
0x00007db0:      0x00000000: SF_CLIP VP: Min extents: 0.00x0.00
0x00007db8:      0x00000000: SF_CLIP VP: Max extents: 299.00x349.00

While here, fix the wrong offsets for the guardband (I didn't check if it used
to be valid on GEN4).

v2: Remove leftover GET_BITS which belongs later in the series. (Topi)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>