i965: allow image_size on float images

i965: allow image_size on float images

This got missed because the piglit test only tested int images to avoid a
combinatiorial explosion of format, targets, stages and sizes which
takes more than 5 minutes to test on nvidia's driver.

This patch also drops the IMAGE_FUNCTION_AVAIL_ATOMIC which is not applicable
to the image_size codepath but was not hurting in any way.

Signed-off-by: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>