i915/sync: Replace prefix 'intel_sync' -> 'intel_gl_sync'

i915/sync: Replace prefix 'intel_sync' -> 'intel_gl_sync'

This is the porting of corresponding patch for i965,
i.e. commit 2516d83 i965/sync: Replace prefix 'intel_sync' -> 'intel_gl_sync'

The only difference compared to i965 one is that intel_check_sync() was renamed
to intel_gl_check_sync() here, as it is more appropriate.

Here follows original commit 2516d83 message by Chad Versace:

"I'm about to implement DRI2_Fenc in intel_syncobj.c.  To prevent
madness, we need to prefix functions for GL_ARB_sync with 'gl' and
functions for DRI2_Fence with 'dri'. Otherwise, the file will become
a jumble of similiarly named functions.

For example:
    old-name:      intel_client_wait_sync()
    new-name:      intel_gl_client_wait_sync()
    soon-to-come:  intel_dri_client_wait_sync()

I wrote this renaming commit separately from the commit that implements
DRI2_Fence because I wanted the latter diff to be reviewable."