drm/i915/fbc: inline intel_fbc_can_choose()

drm/i915/fbc: inline intel_fbc_can_choose()

It only has two checks now, so it makes sense to just move the code to
the caller.

Also take this opportunity to make no_fbc_reason make more sense: now
we'll only list "no suitable CRTC for FBC" instead of maybe giving a
reason why the last CRTC we checked was not selected, and we'll more
consistently set the reason (e.g., if no primary planes are visible).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1478883461-20201-5-git-send-email-paulo.r.zanoni@intel.com