drm/amdgpu: remove ip dump reg_count variable

reg_count is not used and the register count is
directly derived from the array size and hence
removed.

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>