i965: check for GL_TEXTURE_EXTERNAL_OES at miptree_create_for_teximage

i965: check for GL_TEXTURE_EXTERNAL_OES at miptree_create_for_teximage

Forgotten on commit "i965: Fix calculation of the image height at start level".

Thanks to Ilia Mirkin for point it.

Fixes the following regressions on Haswell and Broadwell:
ES2-CTS.gtf.GL2ExtensionTests.egl_image_external.TestSimpleUnassociated (crash back to pass)
ES2-CTS.gtf.GL2ExtensionTests.egl_image_external.TestSimple (crash back to fail)
ES2-CTS.gtf.GL2ExtensionTests.egl_image_external.TestVertexShader (crash back to fail)

https://bugs.freedesktop.org/show_bug.cgi?id=97761

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>