drm/amdkfd: Use resource_size() helper function

Use the resource_size() function instead of a open coded computation
resource size. It makes the code more readable.

Issue identified using resource_size.cocci coccinelle semantic patch.

Signed-off-by: Deepak R Varma <drv@mailo.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>