mesa: Fix texture target validation for glFramebufferTexture()

mesa: Fix texture target validation for glFramebufferTexture()

Previously we were using the code path for validating
glFramebufferTextureLayer().  But glFramebufferTexture() allows
additional texture types.

Fixes piglit tests:
- spec/!OpenGL 3.2/layered-rendering/gl-layer-cube-map
- spec/!OpenGL 3.2/layered-rendering/framebuffertexture

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
v2: Clarify comment above framebuffer_texture().

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>