drm/amdgpu: fix max_entries calculation v4

Calculate the correct value for max_entries or we might run after the
page_address array.

v2: Xinhui pointed out we don't need the shift
v3: use local copy of start and simplify some calculation
v4: fix the case that we map less VA range than BO size

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 1e691e24 drm/amdgpu: stop allocating dummy GTT nodes
Reviewed-by: xinhui pan <xinhui.pan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>