Some enum values such as GLES1's GL_CLIP_PLANE0 and GLES2+'s
GL_CLIP_DISTANCE0_EXT have the same value.
Thus, the enum switch code has to be separated for GLES1 & GLES2+

Bug: angleproject:4481
Change-Id: I2fcb95f3f75f1fe683a072bee02e797a95686480
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2145284
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>