meta: Don't modify GL_GENERATE_MIPMAP state when it doesn't exist

meta: Don't modify GL_GENERATE_MIPMAP state when it doesn't exist

This is a bit of a hack.  _mesa_meta_GenerateMipmap shouldn't even be
used in contexts where GL_GENERATE_MIPMAP doesn't exist (i.e., core
profile and ES2) because it uses fixed-function, and fixed-function
doesn't exist there either!

A GLSL-based _mesa_meta_GenerateMipmap should be available soon.  When
that is available, this patch will be irrelevant and should be reverted.

v2: Change (ctx->API != API_OPENGLES2 && ctx->API != API_OPENGL_CORE) to
(ctx->API == API_OPENGL || ctx->API == API_OPENGLES) based on review
comment from Brian Paul.

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