drm/amd/amdgpu/amdgpu_gart: Correct a couple of function names in the docs

drm/amd/amdgpu/amdgpu_gart: Correct a couple of function names in the docs

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/amdgpu_gart.c:73: warning: expecting prototype for amdgpu_dummy_page_init(). Prototype was for amdgpu_gart_dummy_page_init() instead
 drivers/gpu/drm/amd/amdgpu/amdgpu_gart.c:96: warning: expecting prototype for amdgpu_dummy_page_fini(). Prototype was for amdgpu_gart_dummy_page_fini() instead

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Nirmoy Das <nirmoy.das@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>