drm/amdgpu: allocate entities on demand

Currently we pre-allocate entities and fences for all the HW IPs on
context creation and some of which are might never be used.

This patch tries to resolve entity/fences wastage by creating entity
only when needed.

v2: allocate memory for entity and fences together

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>