drm/i915: don't report compression when fbc is disabled

When i915_fbc_status is read while fbc is disabled,
it reports compressing to be true, which is confusing.
Report compressing only when fbc is enabled.

v2 (from Paulo): commit message capitalization.

Signed-off-by: vathsala nagaraju <vathsala.nagaraju@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474534193-9527-1-git-send-email-vathsala.nagaraju@intel.com