drm/i915: Relocate FBC_LLC_READ_CTRL

In the case of FBC_LLC_READ_CTRL the "FBC" stands for
frame buffer _caching_, not frame buffer compression.
Move the register definition out from the middle of the
frame buffer compression register definitions. Let's
just stick it somewhere with similar looking register
offsets.

And while at it switch it over to REG_BIT().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104144520.22605-15-ville.syrjala@linux.intel.com


Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>