mesa: Fix generic compressed texture formats' handling in glTexImage/glCopyTexImage

mesa: Fix generic compressed texture formats' handling in glTexImage/glCopyTexImage

The generic texture formats should be accepted by the <internalformat>
parameter of TexImage1D, TexImage2D, TexImage3D, CopyTexImage1D, and
CopyTexImage2D functions. When the application specifies a generic
format, the driver is free to pick an uncompressed format.

This patch reverts the changes due to following commit:
commit a36581ccc06693231011c3fe136207e73191b1ce
mesa: do more teximage error checking for generic compressed formats

This patch fixes compressed texture format failures in intel oglconform
pxconv-gettex test case:
https://bugs.freedesktop.org/show_bug.cgi?id=47220

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>