drm/i915: fix comment on I915_{READ, WRITE}_FW

drm/i915: fix comment on I915_{READ, WRITE}_FW

Comment mentioned use of intel_uncore_forcewake_irq{unlock, lock}
functions which are nonexistent (and never were).

The description was also incomplete and could cause confusion. Updated
comment is more elaborate on usage and caveats.

v2: mention __locked variant of intel_uncore_forcewake_{get,put} instead
    of plain ones

Cc: Chris Wilson <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Signed-off-by: Arkadiusz Hiler <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
[Mika: removed two superfluous lines on comment noted by Chris]
Signed-off-by: Mika Kuoppala <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]