i965: Bail on the BLT path if BlitFramebuffer requires sRGB conversion.

i965: Bail on the BLT path if BlitFramebuffer requires sRGB conversion.

Modern OpenGL BlitFramebuffer require sRGB encode/decode when
GL_FRAMEBUFFER_SRGB is enabled.  The blitter can't handle this,
so we need to bail.  On Gen4-5, this means falling back to Meta,
which should handle it.

We allow sRGB <-> sRGB blits, as decode then encode ought to be a noop
(other than potential precision loss, which nobody wants anyway).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>