mesa: Expose function to calculate whether a shader image unit is valid.

mesa: Expose function to calculate whether a shader image unit is valid.

A future commit will remove all texture object-dependent derived state
from the image unit struct to make validation unnecessary on texture
state changes.  Instead of checking gl_image_unit::_Valid drivers will
be required to call this function when needed to find out whether an
image unit is in a valid state and whether access from the shader is
allowed.

Tested-by: Ye Tian <yex.tian@intel.com>
CC: "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>