drm/gem: Fix sphinx warnings

drm/gem: Fix sphinx warnings

Sphinx really wants colons after arguments :/

Fixes the following warnings:
drm_gem.c:1384: warning: Function parameter or member 'fence_array' not described in 'drm_gem_fence_array_add'
drm_gem.c:1384: warning: Function parameter or member 'fence' not described in 'drm_gem_fence_array_add'
drm_gem.c:1435: warning: Function parameter or member 'fence_array' not described in 'drm_gem_fence_array_add_implicit'
drm_gem.c:1435: warning: Function parameter or member 'obj' not described in 'drm_gem_fence_array_add_implicit'
drm_gem.c:1435: warning: Function parameter or member 'write' not described in 'drm_gem_fence_array_add_implicit'

Fixes: 988766346141 ("drm: Add helpers for setting up an array of dma_fence dependencies.")
Cc: Eric Anholt <eric@anholt.net>
Cc: Qiang Yu <yuq825@gmail.com> (v1)
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190424204916.222155-1-sean@poorly.run