lima: make lima_context_framebuffer subtype of pipe_framebuffer_state

lima: make lima_context_framebuffer subtype of pipe_framebuffer_state

Currently the lima driver saves the framebuffer state in its
from-scratch struct lima_context_framebuffer. However, util_blitter
requires to save framebuffer with standard struct
pipe_framebuffer_state.

Make the lima_context_framebuffer a subtype of the standard
pipe_framebuffer_state, thus the standard part can be used for
util_blitter framebuffer state saving.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Qiang Yu <yuq825@gmail.com>