lima: drop scissor-based partial clear in lima_clear()

According to src/gallium/docs/source/context.rst, the clear() function
should not do partial clear of a surface -- it only does clear on one or
more full surface. In addition, the partial clear code in lima_clear
seems to be the reason of some weird GPIR error.

Only do full framebuffer clear in lima_clear() now, to correct the
behavior and prevent the GPIR error.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
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merge request