drm/amdgpu: get absolute offset from doorbell index

This patch adds a helper function which converts a doorbell's
relative index in a BO to an absolute doorbell offset in the
doorbell BAR.

V2: No space between the variable name doc (Luben)

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>