egl/dri2: remove error checks on return values from mtx_lock and cnd_wait

egl/dri2: remove error checks on return values from mtx_lock and cnd_wait

This removes unnecessary error checks on return result of mtx_lock
and cnd_wait calls as in all other places in MESA source since there
is no chance that any of these functions return any of error codes
in current implementation.

This patch also removes a redundent _eglError call that follows
EGL_FALSE check in the bottom of dri2_client_wait_sync.

Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>