drm/i915: set dev_priv->fbc.crtc before scheduling the enable work

drm/i915: set dev_priv->fbc.crtc before scheduling the enable work

This thing where we need to get the crtc either from the work
structure or the fbc structure itself is confusing and unnecessary.
Set fbc.crtc right when scheduling the enable work so we can always
use it.

The problem is not what gets passed and how to retrieve it. The
problem is that when we're in the other parts of the code we always
have to keep in mind that if FBC is already enabled we have to get the
CRTC from place A, if FBC is scheduled we have to get the CRTC from
place B, and if it's disabled there's no CRTC. Having a single place
to retrieve the CRTC from allows us to treat the "is enabled" and "is
scheduled" cases as the same case, reducing the mistake surface. I
guess I should add this to the commit message.

Besides the immediate advantages, this is also going to make one of
the next commits much simpler. And even later, when we introduce
enable/disable + activate/deactivate, this will be even simpler as
we'll set the CRTC at enable time. So all the
activate/deactivate/update code can just look at the single CRTC
variable regardless of the current state.

v2: Improve commit message (Chris).
v3: Rebase after changing the patch order.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/