drm/i915: Get the correct wakeref for reading HOTPLUG_EN et al.

Without it we get:
 Unclaimed read from register 0x1e1110
 WARNING: CPU: 2 PID: 1029 at drivers/gpu/drm/i915/intel_uncore.c:1101 __unclaimed_reg_debug+0x40/0x50 [i915]
 Call Trace:
  fwtable_read32+0x233/0x300 [i915]
  i915_interrupt_info+0xa73/0xd60 [i915]
  seq_read+0xdb/0x3c0
  full_proxy_read+0x51/0x80
  vfs_read+0x9e/0x160
  ksys_read+0x8f/0xe0
  do_syscall_64+0x55/0x1c0
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109824


Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190912125418.23115-2-arkadiusz.hiler@intel.com