i965: fix the constant interp bitmask for flat mode

i965: fix the constant interp bitmask for flat mode

Fix the constant interpolation enable bit mask for flat light mode.
FRAG_BIT_COL0 attribute bit might be 0, in which case we need to
shift one more bit right.

This would fix the oglc specularColor test fail on both Sandybridge and
Ivybridge.

v2: move the constant interp bitmask setup code into for(; attr <
FRAG_ATTRIB_MAX; attr++) loop suggested by Eric.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Xiang, Haihao <haihao.xiang@intel.com>