drm/amd: declare amdgpu_exp_hw_support in amdgpu.h

Declare `amdgpu_exp_hw_support` as extern in amdgpu.h to address the
following sparse warning:
drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c:118:5: warning: symbol 'amdgpu_exp_hw_support' was not declared. Should it be static?

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Suggested-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>