drm: Fix warning with make xmldocs caused by drm_irq.c

This patch fix following warnings.

Warning(.//drivers/gpu/drm/drm_irq.c:1279): No description
found for parameter drm_crtc'
Warning(.//drivers/gpu/drm/drm_irq.c:1279): Excess function
parameter 'crtc' description in 'drm_crtc_vblank_reset'

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