GBE: optimize read_image to avoid get sampler info dynamically.

GBE: optimize read_image to avoid get sampler info dynamically.

Most of time, the user is using a const sampler value in the kernel
directly. Thus we don't need to get the sampler value through a function
call. And this way, the compiler front end could do much better optimization
than using the dynamic get sampler information. For the luxmark's
median/simple case, this patch could get about 30-45% performance gain.

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>
Reviewed-by: "Yang, Rong R" <rong.r.yang@intel.com>