Fixup GL instanced rendering commands

We've ended up with a flag in both GrCaps and GrGLCaps to indicate
the same instanced rendering functionality. This change removes the
GrGLCaps version of the flag and also also fixes the interface assembler
to use the proper suffix when loading the EXT version of the extension.

BUG=skia:

Review URL: https://codereview.chromium.org/1537483002
3 files changed