mesa/i965: Refactor brw_is_front_buffer_{drawing,reading} to common code

mesa/i965: Refactor brw_is_front_buffer_{drawing,reading} to common code

There are multiple similar implementations of these functions, and a
later patch was going to add another.

v2: Move removing intel_framebuffer to a different patch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>