drm/i915: add SW tracking to FBC enabling

Currently, calling intel_fbc_enabled() will trigger a register read.
And we call it a lot of times, even when FBC is disabled, so saving a
few cycles would be a good thing.

Another reason for this patch is because we currently call
intel_fbc_enabled() while the HW is runtime suspended, so the read
makes no sense and triggers a WARN. This happens even if FBC is
disabled by default. Of course one could argue that we just shouldn't
be calling intel_fbc_enabled() while the driver is runtime suspended,
and I agree that's a good argument, but I still think that the reason
explained in the first paragraph already justifies the patch.
This problem can easily be reproduced with many subtests of
igt/pm_rpm, and it is a regression introduced by:

    commit c5ad011d
    Author: Rodrigo Vivi <rodrigo.vivi@intel.com>
    Date:   Mon Aug 4 03:51:38 2014 -0700
        drm/i915: FBC flush nuke for BDW

Testcase: igt/pm_rpm/cursor (and others)
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>