drm/amd/pm: remove unused declarations in hardwaremanager.h

phm_is_hw_access_blocked() and phm_block_hw_access() has been
removed since commit 698f88e6 ("drm/amd/powerplay: delete
dead code in powerplay"), so remove them.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913024847.552254-6-cuigaosheng1@huawei.com