The tiled memcpy doesn't work for copying from RGBX to RGBA because it
doesn't override the alpha component to 1.0. Commit 2cebaac4

 added
a check to disable it for RGBX formats by looking at the TexFormat.
However a lot of the rest of the code base is written with the
assumption that an RGBA texture can be used internally to implement a
GL_RGB texture. If that is done then this check breaks. This patch
makes it instead check the base format of the texture which I think
more directly matches the intention.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>