mesa: inline vbo_context inside gl_context to remove vbo_context dereferences

mesa: inline vbo_context inside gl_context to remove vbo_context dereferences

The number of lines in the disassembly of vbo_exec_api.c.o decreased
by 4.5%, which roughly corresponds to a decrease in instructions
for immediate mode thanks to the removal of ctx->vbo_context dereferences.

It increases performance in one Viewperf11 subtest by 2.8%.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4314>