i965: Check base format to determine whether to use tiled memcpy

i965: Check base format to determine whether to use tiled memcpy

The tiled memcpy doesn't work for copying from RGBX to RGBA because it
doesn't override the alpha component to 1.0. Commit 2cebaac479d4 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>