drm/lima: fix recovering from PLBU out of memory

It looks like on PLBU_OUT_OF_MEM interrupt we need to resume from where we
stopped, i.e. new PLBU heap start is old end. Also update end address
in GP frame to grow heap on 2nd and subsequent out of memory interrupts.

Fixes: 2081e8dc ("drm/lima: recover task by enlarging heap buffer")
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200215035026.3180698-1-anarsoul@gmail.com