egl: Simplify the "driver" interface

"Driver" isn't a great word for what this layer is, it's effectively a
build-time choice about what OS you're targeting. Despite that both of
the extant backends totally ignore the display argument, the old code
would only set up the backend relative to a display.

That causes problems! One problem is it means eglGetProcAddress can
generate X or Wayland protocol when it tries to connect to a default
display so it can call into the backend, which is, you know, completely
bonkers. Any other EGL API that doesn't reference a display, like
EGL_EXT_device_query, would have the same issue.

Fortunately this is a problem that can be solved with the delete key.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>

docs/egl.html

@@ -130,16 +130,6 @@ mesa/demos repository.</p>
 runtime</p>
 
 <dl>
-<dt><code>EGL_DRIVER</code></dt>
-<dd>
-
-<p>This variable specifies a full path to or the name of an EGL driver.  It
-forces the specified EGL driver to be loaded.  It comes in handy when one wants
-to test a specific driver.  This variable is ignored for setuid/setgid
-binaries.</p>
-
-</dd>
-
 <dt><code>EGL_PLATFORM</code></dt>
 <dd>