drm/i915: Fix warnings while make xmldocs caused by intel_lrc.c

drm/i915: Fix warnings while make xmldocs caused by intel_lrc.c

This patch fix following warnings while "make xmldocs".
.//drivers/gpu/drm/i915/intel_lrc.c:780: warning: No description
found for parameter 'req'
.//drivers/gpu/drm/i915/intel_lrc.c:780: warning: Excess function
 parameter 'request' description in 'intel_logical_ring_begin'
.//drivers/gpu/drm/i915/intel_lrc.c:780: warning: Excess function
 parameter 'ctx' description in 'intel_logical_ring_begin'

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>