mesa: Validate original image internal format rather than derived mesa format.

mesa: Validate original image internal format rather than derived mesa format.

This matches what _mesa_BindImageTextures() does.  The derived image format
(gl_texture_image::TexFormat) isn't necessarily equivalent to the internal
format of the texture image.  If a forbidden internal format has been
specified we need to mark the image unit as invalid as required by the spec,
regardless of the derived format.  Fixes the "invalid"
ARB_shader_image_load_store piglit test.

Reviewed-by: Matt Turner <mattst88@gmail.com>